the Jester
Legend
I love great big megadungeons; in my 5e game, the local megadungeon has proven to be the most popular place to adventure for both groups of pcs.
I'm pretty solidly of the "adventures set in a megadungeon aren't about clearing out the megadungeon" mindset. It's important that the pcs be able to choose a goal (even if it's just "explore more of the dungeon") that gives them a sense of "we made it" without having to clear 400+ rooms of monsters, traps, etc.
One thing that's really important (to me) in this kind of environment is a variety of encounter types. Some monsters, some traps, the rare free treasure, dungeon hazards, weird magic pools, chances to talk to strange monster/npc types, etc.
I'm pretty solidly of the "adventures set in a megadungeon aren't about clearing out the megadungeon" mindset. It's important that the pcs be able to choose a goal (even if it's just "explore more of the dungeon") that gives them a sense of "we made it" without having to clear 400+ rooms of monsters, traps, etc.
One thing that's really important (to me) in this kind of environment is a variety of encounter types. Some monsters, some traps, the rare free treasure, dungeon hazards, weird magic pools, chances to talk to strange monster/npc types, etc.