Jahydin
Hero
Understand where you're coming from.In my 43 years of DMing experience, I have learned this about megadungeons - there is no way to make them fun. They are repetitive, dull, mapping and resource management are chores that players have no interest in.
This is what I have learned: keep it varied, mix it up, have a little bit of of dungeon, have a little bit of wilderness, have a little bit of social, have a little bit of everything else you can think of.
In my games, the mega-dungeon would not be the adventure. It's just a very enticing place to explore when the player's feel like it. That said, it better be worth it! Extra deadly, but rewards great enough everyone around town whispers about them.
I said "would" because I've never ran anything on the scale of Barrowkeep. Truthfully, when I wrote my post I had the standard 3-level old school dungeons in mind since most "dungeons' now are pretty tiny, like 5 rooms.