Here's my two cents:
The guidelines in the DMG assume a base game experience; no multi-classing, no feats, no flanking, rock stock basic rules no variants. Given that assumption, I don't think the DMG guidelines are too far off, math-wise, given the assumption of basically stock PCs whose individual abilities are thematically and mechanically linked. Once you start adding in feats, multi-classing, etc., versatility of individual PCs starts to make a significant impact on how challenging an encounter will be. The rogue is only going to get one strong attack in a round with, the cleric won't be dropping fireballs, the wizard won't be busting out cure spells, the fighter will be taking most of the damage. In such a case, the math for the encounter guidelines hold up; the fighter is going to go down significantly quicker with each additional creature attacking it, the cleric will have to heal more often, reducing their time spent attacking and dealing damage, the wizard will have to use more controller effects, rogue can't just stand up in melee opposite the tank without getting attacked as well.
However, I don't think many of us play stock games. We play with feats, multi-classing, variant and other homebrew rules and rulings which fundamentally change the math of the system. Consider a feat like healer; 1d6+4+targetlevel HP recovery 1/rest with an infinite stabilize to 1 option. That's a significant increase in available healing, and an absurd increase to a party's ability to keep PCs up and in the fight. A party of 4th level PCs, assuming two rests a day, and a 20 gp investment in healing kits (just have each PC buy a healing kit and keep it on their belt, the guy with the healer feat uses each PC's kit to determine uses), is going to see 3 uses of healer healing per day, for 3d6 (9) + 12 +12, or 33 additional healing a day, and for a party of 4th level characters, that's pretty close to max HP for around 3 of the PCs, and doesn't consider any stabilize to 1 uses. That's better than a 3rd level healing spell from a life cleric with an 18 wisdom (and the party doesn't even have 3rd level spells yet, 3d8+4+2+3, or 12+9 or 21 on average, it's not until 5th level spells with a 20 wisdom that it even starts to catch up to healer with 5d8+5+2+5 for 32, since healer actually increases by 9 over the course of the day at that point assuming 7th level party to have access to 5th level magic). Another example being alert, adding +5 to initiative and can't be surprised, that's effectively advantage on initiative, which enables a lot of characters to deal extra damage which changes the dynamics of a fight, eating through monster resources faster. Healer makes monsters less effective, Alert makes PCs more effective, and a lot of feats work along similar lines. Heck, consider skulker, no disadvantage in dim-light means a character with darkvision and skulker has advantage against every other character, even if they have darkvision, in the darkness. Got an underdark campaign? That character is vastly more effective now with a small investment cost.
Now, a multi-class character isn't going to be so much more powerful than a normal character as to burn through monsters faster, but the added versatility diminishes the effectiveness of various options you may wish to choose. If the fighter takes a few levels of the UA Revised Ranger, now they're basically immune to difficult terrain, letting them get to and start dealing damage sooner than a straight fighter. A sorcerer who is a favored soul origin is potentially going to have access to healing and buff spells that create powerful synergy that makes the party as a whole more efficient; the cleric is out of spells? Let me just eat a slot with a bonus action for the sorcery points and then twin this healing spell so that I can heal the tank and myself, letting us keep fighting while the cleric switches to offensive cantrips and helps kill the monsters quicker. There are plenty of examples where a MCd character has interesting options that allow for solid efficiency increases. Heck, a sorc/bard is going to have no less than 6 cantrips with 1/1 level investment. That's a LOT of cantrips covering a LOT of situations. Those kinds of options reduce the difficulty of various challenges.
My point with this blurb is that if you ARE going to retool the encounter building tables, I suggest that you do create different sets which create different guidelines depending on what options you are using in your game. A rock stock game, I think, could be fun and challenging using the encounter building tools in the DMG, but a game with feats, MCs, splat options (UA stuff I'm looking at you), are going to find these encounters too simple. So I think what you should focus on, rather than trying to create a new base guideline, is to generate a guideline based on the assumption that you're allowing feats and MC. A game that allows GWM is going to find melee encounters easier to handle than a group that doesn't. A group that allows MC is going to handle diversified challenges easier (unless the group focuses on a particular aspect, e.g. melee combat optimization, ranged combat optimization, social optimization, exploration optimization, etc.). As such, your efforts, I feel, would be best focused towards a table for DMs who allow these options, with some text guidelines explaining how to build more challenging encounters without resorting to throwing absurd CR creatures at your party. For these tables, I don't think any multipliers should be used, at all. I think the tables should be derived by determining easy, medium, hard, deadly for an individual character, and then total encounter budget is the sum of individual budgets for each party member.
My reasoning for removing the multipliers is that they are too wary of the action economy of lots of monsters; the assumption is that the monsters will have so many extra actions that you'll overwhelm the PCs, where I don't think that is the case in games that employ options like feats and MC. The monsters were created and tuned under the assumption that they would be used against stock PCs, and in that case, adding more monsters is going to make it harder because they aren't going to be as diversified, and you aren't going to see as powerful of novas from your PCs (not that they can't, it just isn't going to be as strong, a raging barb rocking a greataxe is going to be dealing 20 damage less without feats than a barb in a game that does allow feats, since it would be stupid not to take GWM in such a game with such a character). In a game with feats and MCs, extra actions from team monster basically means very little.
I think the others are right when they suggest the bulk of the separation occurring around level 8, that's when feats and MC really start to affect the game in a significant fashion, but even at 3rd level, my UA RRanger/Sorc was very effective in a variety of combat situations (can melee, range, and lots of skills), so I would suggest starting the separation at a lower level, but with diminished potency, which increases moreso at level 8 and beyond. I think low levels are just as much of a problem as higher levels, the numbers are just so much lower all around that dice play a significant role in adjudication of actions that even a single lucky roll can drastically influence the outcome of an encounter.
I think also that you should include verbiage strongly cautioning against using a single enemy, especially once the party hits level 5 and start getting multi-attack (extra attack, extra cantrip dice, EB rays, etc.) since the PCs will have so many more actions. Yes, this eats up budget for lower CR monsters, but it would rebalance the action economy, letting higher CR monsters actually feel like greater threats. I mean, you're making this for non-noob DMs, so they should know this, but we all know what happens when you assume. A lot of combat balance issues are solved by not using single creature encounters.
I guess, really, I should reword my proposal and walk back something I said earlier; there should be a multiplier, and it applies if you absolutely insist upon using a single monster, and it doesn't change the effective encounter XP, but actually increases the XP budget. Say, a 1.5x increase in budget if you are using only a single creature for the encounter.
I find that when I run a single monster, it tends to go down rather quickly unless I have a way to reasonably fiat immunity from being targeted. For instance, a succubus can just kind of bounce from the material plane and pop back in when it wants, kind of useful for letting it last more than a round or two. Whereas if I take a single monster I wanted to use as a threat, and then throw in a bunch of lower CR minions, it tends to fare much better because a good chunk of the party can't just sit there with enemies on it and still effectively deal damage and maintain a state of consciousness without burning up resources or getting help from the other players who would otherwise be burning down the big bad.
I, personally, wouldn't exclude the low CR monsters from the total encounter budget, to be perfectly honest, because once you divide the XP up among the party, they really amount to trivial amounts anyway, so they're barely eating through your budget in the first place, and, with a correctly tuned for options table, you'll have the extra budget in the first place.
Further, sort of off but still on-topic note: CR is whack when you take into account the optional rules that most groups are playing with. Unless we can come up with a new CR table for the monsters that is formed under the assumption that PCs will have access to feats and MC, I think your table is going to run into problems if it is the result of formulaic manipulation. I think if you want this to work in the long run, you'd need to look at the classes and builds, and see at what levels the power tends to jump up for PCs, and adjust the table to allow for greater budget at those levels. WIth casters, it's basically a solid power bump ever odd level, but with your melee it's more like a significant bump every tier, and your utility classes tend to have a solid power creep across all levels. As such, I think I would make my adjustments slightly more pronounced on the odd levels, with a more powerful adjustment every 4th or 5th level, in addition to an overall upwards adjustment.
I think you could mute the over-all effect of the upwards adjustment by changing the way the difficulty categories are handled; assume the base XP budget is an "easy" encounter, or rather, a baseline. Not sure on exact numbers, so my example isn't really based on anything, but, if you have a budget of 1500 for easy, then medium would be 1.5x that budget at 2250, hard would be 3x for 4500, deadly would be a 5x budget for 7500. Again, those are numbers I pulled from betwixt my cheeks, so the specific numbers aren't important, but I think they illustrate basically what I'm trying to say, though I feel that the multipliers are probably close to what they would end up at. If this 1500 base budget were for a party of 5 3rd level characters, a deadly encounter for them would be a CR 9 and ~12-13 CR 1 creatures, which to me, sounds fairly deadly, but not impossible, and you'll probably have a character get dropped to 0, if not outright killed. It could also be ~4 CR 5 creatures, ~11 CR 3 creatures, so on and so forth. I don't use the base encounter table regularly, so I don't know how the numbers I made up compare to the base table, I saw that the encounters I built tend to wind up too easy thanks to my reliance on hordes of low CR monsters to prop-up the tougher enemies, which I find tends to work well to eat up PC resources and keep encounters interesting.
If'n I were to undertake this operation, I think I'd quit, so I just want to say grats on keeping up the effort, and good work so far. I'm glad to see how open to suggestion you are, and I hope I didn't waste your time with my semi-coherent rambling.
The guidelines in the DMG assume a base game experience; no multi-classing, no feats, no flanking, rock stock basic rules no variants. Given that assumption, I don't think the DMG guidelines are too far off, math-wise, given the assumption of basically stock PCs whose individual abilities are thematically and mechanically linked. Once you start adding in feats, multi-classing, etc., versatility of individual PCs starts to make a significant impact on how challenging an encounter will be. The rogue is only going to get one strong attack in a round with, the cleric won't be dropping fireballs, the wizard won't be busting out cure spells, the fighter will be taking most of the damage. In such a case, the math for the encounter guidelines hold up; the fighter is going to go down significantly quicker with each additional creature attacking it, the cleric will have to heal more often, reducing their time spent attacking and dealing damage, the wizard will have to use more controller effects, rogue can't just stand up in melee opposite the tank without getting attacked as well.
However, I don't think many of us play stock games. We play with feats, multi-classing, variant and other homebrew rules and rulings which fundamentally change the math of the system. Consider a feat like healer; 1d6+4+targetlevel HP recovery 1/rest with an infinite stabilize to 1 option. That's a significant increase in available healing, and an absurd increase to a party's ability to keep PCs up and in the fight. A party of 4th level PCs, assuming two rests a day, and a 20 gp investment in healing kits (just have each PC buy a healing kit and keep it on their belt, the guy with the healer feat uses each PC's kit to determine uses), is going to see 3 uses of healer healing per day, for 3d6 (9) + 12 +12, or 33 additional healing a day, and for a party of 4th level characters, that's pretty close to max HP for around 3 of the PCs, and doesn't consider any stabilize to 1 uses. That's better than a 3rd level healing spell from a life cleric with an 18 wisdom (and the party doesn't even have 3rd level spells yet, 3d8+4+2+3, or 12+9 or 21 on average, it's not until 5th level spells with a 20 wisdom that it even starts to catch up to healer with 5d8+5+2+5 for 32, since healer actually increases by 9 over the course of the day at that point assuming 7th level party to have access to 5th level magic). Another example being alert, adding +5 to initiative and can't be surprised, that's effectively advantage on initiative, which enables a lot of characters to deal extra damage which changes the dynamics of a fight, eating through monster resources faster. Healer makes monsters less effective, Alert makes PCs more effective, and a lot of feats work along similar lines. Heck, consider skulker, no disadvantage in dim-light means a character with darkvision and skulker has advantage against every other character, even if they have darkvision, in the darkness. Got an underdark campaign? That character is vastly more effective now with a small investment cost.
Now, a multi-class character isn't going to be so much more powerful than a normal character as to burn through monsters faster, but the added versatility diminishes the effectiveness of various options you may wish to choose. If the fighter takes a few levels of the UA Revised Ranger, now they're basically immune to difficult terrain, letting them get to and start dealing damage sooner than a straight fighter. A sorcerer who is a favored soul origin is potentially going to have access to healing and buff spells that create powerful synergy that makes the party as a whole more efficient; the cleric is out of spells? Let me just eat a slot with a bonus action for the sorcery points and then twin this healing spell so that I can heal the tank and myself, letting us keep fighting while the cleric switches to offensive cantrips and helps kill the monsters quicker. There are plenty of examples where a MCd character has interesting options that allow for solid efficiency increases. Heck, a sorc/bard is going to have no less than 6 cantrips with 1/1 level investment. That's a LOT of cantrips covering a LOT of situations. Those kinds of options reduce the difficulty of various challenges.
My point with this blurb is that if you ARE going to retool the encounter building tables, I suggest that you do create different sets which create different guidelines depending on what options you are using in your game. A rock stock game, I think, could be fun and challenging using the encounter building tools in the DMG, but a game with feats, MCs, splat options (UA stuff I'm looking at you), are going to find these encounters too simple. So I think what you should focus on, rather than trying to create a new base guideline, is to generate a guideline based on the assumption that you're allowing feats and MC. A game that allows GWM is going to find melee encounters easier to handle than a group that doesn't. A group that allows MC is going to handle diversified challenges easier (unless the group focuses on a particular aspect, e.g. melee combat optimization, ranged combat optimization, social optimization, exploration optimization, etc.). As such, your efforts, I feel, would be best focused towards a table for DMs who allow these options, with some text guidelines explaining how to build more challenging encounters without resorting to throwing absurd CR creatures at your party. For these tables, I don't think any multipliers should be used, at all. I think the tables should be derived by determining easy, medium, hard, deadly for an individual character, and then total encounter budget is the sum of individual budgets for each party member.
My reasoning for removing the multipliers is that they are too wary of the action economy of lots of monsters; the assumption is that the monsters will have so many extra actions that you'll overwhelm the PCs, where I don't think that is the case in games that employ options like feats and MC. The monsters were created and tuned under the assumption that they would be used against stock PCs, and in that case, adding more monsters is going to make it harder because they aren't going to be as diversified, and you aren't going to see as powerful of novas from your PCs (not that they can't, it just isn't going to be as strong, a raging barb rocking a greataxe is going to be dealing 20 damage less without feats than a barb in a game that does allow feats, since it would be stupid not to take GWM in such a game with such a character). In a game with feats and MCs, extra actions from team monster basically means very little.
I think the others are right when they suggest the bulk of the separation occurring around level 8, that's when feats and MC really start to affect the game in a significant fashion, but even at 3rd level, my UA RRanger/Sorc was very effective in a variety of combat situations (can melee, range, and lots of skills), so I would suggest starting the separation at a lower level, but with diminished potency, which increases moreso at level 8 and beyond. I think low levels are just as much of a problem as higher levels, the numbers are just so much lower all around that dice play a significant role in adjudication of actions that even a single lucky roll can drastically influence the outcome of an encounter.
I think also that you should include verbiage strongly cautioning against using a single enemy, especially once the party hits level 5 and start getting multi-attack (extra attack, extra cantrip dice, EB rays, etc.) since the PCs will have so many more actions. Yes, this eats up budget for lower CR monsters, but it would rebalance the action economy, letting higher CR monsters actually feel like greater threats. I mean, you're making this for non-noob DMs, so they should know this, but we all know what happens when you assume. A lot of combat balance issues are solved by not using single creature encounters.
I guess, really, I should reword my proposal and walk back something I said earlier; there should be a multiplier, and it applies if you absolutely insist upon using a single monster, and it doesn't change the effective encounter XP, but actually increases the XP budget. Say, a 1.5x increase in budget if you are using only a single creature for the encounter.
I find that when I run a single monster, it tends to go down rather quickly unless I have a way to reasonably fiat immunity from being targeted. For instance, a succubus can just kind of bounce from the material plane and pop back in when it wants, kind of useful for letting it last more than a round or two. Whereas if I take a single monster I wanted to use as a threat, and then throw in a bunch of lower CR minions, it tends to fare much better because a good chunk of the party can't just sit there with enemies on it and still effectively deal damage and maintain a state of consciousness without burning up resources or getting help from the other players who would otherwise be burning down the big bad.
I, personally, wouldn't exclude the low CR monsters from the total encounter budget, to be perfectly honest, because once you divide the XP up among the party, they really amount to trivial amounts anyway, so they're barely eating through your budget in the first place, and, with a correctly tuned for options table, you'll have the extra budget in the first place.
Further, sort of off but still on-topic note: CR is whack when you take into account the optional rules that most groups are playing with. Unless we can come up with a new CR table for the monsters that is formed under the assumption that PCs will have access to feats and MC, I think your table is going to run into problems if it is the result of formulaic manipulation. I think if you want this to work in the long run, you'd need to look at the classes and builds, and see at what levels the power tends to jump up for PCs, and adjust the table to allow for greater budget at those levels. WIth casters, it's basically a solid power bump ever odd level, but with your melee it's more like a significant bump every tier, and your utility classes tend to have a solid power creep across all levels. As such, I think I would make my adjustments slightly more pronounced on the odd levels, with a more powerful adjustment every 4th or 5th level, in addition to an overall upwards adjustment.
I think you could mute the over-all effect of the upwards adjustment by changing the way the difficulty categories are handled; assume the base XP budget is an "easy" encounter, or rather, a baseline. Not sure on exact numbers, so my example isn't really based on anything, but, if you have a budget of 1500 for easy, then medium would be 1.5x that budget at 2250, hard would be 3x for 4500, deadly would be a 5x budget for 7500. Again, those are numbers I pulled from betwixt my cheeks, so the specific numbers aren't important, but I think they illustrate basically what I'm trying to say, though I feel that the multipliers are probably close to what they would end up at. If this 1500 base budget were for a party of 5 3rd level characters, a deadly encounter for them would be a CR 9 and ~12-13 CR 1 creatures, which to me, sounds fairly deadly, but not impossible, and you'll probably have a character get dropped to 0, if not outright killed. It could also be ~4 CR 5 creatures, ~11 CR 3 creatures, so on and so forth. I don't use the base encounter table regularly, so I don't know how the numbers I made up compare to the base table, I saw that the encounters I built tend to wind up too easy thanks to my reliance on hordes of low CR monsters to prop-up the tougher enemies, which I find tends to work well to eat up PC resources and keep encounters interesting.
If'n I were to undertake this operation, I think I'd quit, so I just want to say grats on keeping up the effort, and good work so far. I'm glad to see how open to suggestion you are, and I hope I didn't waste your time with my semi-coherent rambling.
