When we were playing AD&D 2e, we had so many houserules we practically had written it into another edition. Some of them were eerily prescient of some changes that would happen in 3rd and later editions, actually.
Interestingly enough, these rules tended to cross-pollinate a LOT. I think I only played stock, core, RAW 2e once. Pretty much every group I played used these rules, because the various groups I was in or knew of heard about rules from each other and adopted each other's rules a lot.
From what I can recall (I haven't played 2e since circa 2001). . .
We had a NWP you could get called "Self Improvement", it was extremely popular, and a precursor to the later rule about raising an ability score every four levels. With the Self Improvement NWP, you rolled 5d10 every time you went up in level, and added that as a percentage to your Prime Requisite (if your class had multiple prime requisites, or you were multiclassed, you had to split them evenly among your various Prime Requisites). When that percentage got to 100%, that ability score went up 1 point.
We had a homebrew "Psionic Fighter" class that was pretty much the later Psychic Warrior class, certainly in spirit. I don't remember the actual rules "crunch" for it, but the concept was the same.
We were MUCH more permissive about multiclassing than the core rules. Humans could multiclass. Just about any multiclass combination was allowed, except nobody could get more than 3 classes, you could only have one class of a "group" (Warrior, Mage, Priest, Rogue, Psionicist groups), and you could only have that class in your multiclass character if the race could even be that class to begin with (no Dwarf Fighter/Wizards or Halfling Paladin/Wizards for example). You could add an extra class and become multiclass at any time, as long as you spent downtime training for your new class. You could also "bar" a class at any time and stop progressing in it permanently (undoing this to resume progressing would take a Wish). It was clearly a precursor to the 3e multiclassing system.
We had a very elaborate system for researching spells. It's been about 20 years, so I certainly don't remember the specifics, but we had tables and modifiers and adjustments and costs. It required libraries that were broken down in great detail as to what spell levels they could research, of what schools of magic, and particularly good or bad libraries could give other modifiers to rolls for costs, time, and chances of success on the research. I kind of miss that system, we put a lot of effort into playing Wizards.
Another Wizard rule we had was that you could try to alter spells on the fly by changing their material components. Instead of just using a pinch of sand for a Sleep spell, you might try using sand from a blasted battlefield on the plane of Baator, or from the beaches of the Silver Sea of the Plane of Celestia. You might try using a feather from some magical bird or creature in an Identify spell. The process involved making a Spellcraft NWP check, with modifiers from the DM on how appropriate or inappropriate the substitution was, and then the spell would work (or fizzle if the Spellcraft check was a critical failure or the substitution was completely inappropriate) and the spell would be modified as appropriate. Sometimes it was simply thematic or roleplaying (adding sand from a lower plane into a sleep spell made the sleep filled with tormenting nightmares, I remember that one), sometimes it was very handy (using dung from a fiend instead of bat guano made Fireballs ignore Magic Resistance and fire immunities for fiends). Once a Wizard had set up a component substitution/modification with a successful Spellcraft check, it worked the same for them from then on, but any other wizards would have to make the check to see if they could get it to work like that.
We had some pretty elaborate critical hit and critical fumble tables. If you rolled a 1 or 20 to hit, you then had to start rolling percentages. I've long since lost those tables, but low rolls were bad for you, high rolls were bad for people around you (or your enemies) and towards the middle was neutral (just a miss for a critical miss, just double damage for a critical hit).
I knew a few groups that also had a secret "Destiny' ability score and a system of "hidden talents" that used it. When you created a character, the DM would secretly create that ability score on a straight 3d6 roll and note it. The player didn't know the Destiny score for his character. This score was used for figuring out what character in a group would have something awesome or terrible happen to them. If the campaign plot needed a PC to be a "chosen one", it would be whoever had the highest Destiny score. If a turn of events in an adventure needed something bad for someone, he'd roll Destiny checks for everyone, and whoever failed it the worst would have have that bad thing happen to them.
Also, when characters were created, the DM would roll percentage for each character. If he rolled under their Destiny score, the character had some secret talent or hidden power that even they may not have known about. I never saw the tables, most DM's made up their own based on what they saw/heard from others, usually with dozens of items on the tables of various secret abilities people could learn they had but some examples of these abilities I knew of:
- Prodigy. You're innately talented at something. You got a big bonus to one of your NWP's (or one of your rogue skills if you were a Thief or Bard). I saw a campaign convert this to 3e and it became just a free Skill Focus feat in a skill you have ranks in.
- Personal attention of a God. For whatever reason, this character has the direct, personal attention of a randomly determined deity of the campaign. This could be a HUGE blessing, like if they were a Cleric of that God, or even a worshipper or even if it was just someone to who that Deity was relevant (like a fighter having the God of War's attention). It could also be a HUGE curse, if you had the personal attention of the God of Necromancy or the God of Plagues and you were a Paladin or good-aligned Cleric.
- Naturally talented with weapons. A warrior-group character with this talent was considered proficient with ALL weapons. Other characters got double their starting weapon proficiencies and got an extra weapon proficiency at each level.
- Immortality. You stop aging at adulthood and never normally die of old age. You could only age by supernatural means, such as casting Wish or Limited Wish spells.
- Immune to negative energy. You're naturally pale skinned, always feel cold, people have always seen you as a little creepy, and for some reason that might be tied to that but nobody can ever explain exactly why, you're absolutely immune to level drain, cause wounds spells, and necromantic death magic. You heal through positive energy normally.
- Exempt from Level Limits. This was a Demihuman-only one, you were completely exempt from the level limit in your class. If you were multiclassed, it would be in whatever class you had the highest prime requisite score, if tied, it was randomly rolled.
If you didn't find out in game, or it was something so obvious that you'd know about (like the weapon proficiencies), these talents were fairly easy to unearth through divination. True Seeing would show something odd about someone in their aura and the caster could get a strong clue as to what their talent was via a Wisdom check. The Divination spell would give very strong clues, or confirm a guess if characters suspected something. Contact Other Plane would say what a talent was (if the spell gave an accurate answer), and Commune would say what it is if someone asked (via the clause about a short phrase answer being appropriate instead of yes or no).
