Ok, so megadungeons are fun, right?
But the problem is you have to tell your players like "This is where the action is, ok? Go in the dungeon a lot or else I've prepped all this shit for nothing."
And, in general, dungeons in a sandbox are like that: on the one hand you wanna dream crazy stacking dreams upon dreams, on the other hand, the players have to have the freedom to go wherever they want.
So often, instead of a big dungeon you end up with lots of little ones.
Now as you may or may not know, my solution to this in my game is that, just beneath a thin layer of opportunistic plants, my whole world's just solid dungeon. One big one.
The little ones all feed into the big one, like so...
So like ok first of all you know how one dungeon can have two geographically disparate entrances...
...so then now imagine that points A and B below represent the location of two different small dungeons...
And both sit above (and are wrappped completely around) a level 2 that is smaller than this vast level 1, but which is accessible from many places on that big level 1.
And the still-vast level 2 leads (through various different exits) to a smaller level 3, and below that, a smaller level 4 etc etc on and on until at level 9 you have a really small dungeon with like Demogorgon and 3000 horribles and treasure and godhood.
Basically this allows each "separate" dungeon on the surface to maintain it's own identity (this is the lava dungeon, this is the undead dungeon, this is the dungeon under the mugwump castle...) while cutting down on your prep time as GM--you only have to make one big dungeon with lots of subgenrefied and differentiated "feeders".
There's no reason players even have to know this--they can go in, kill some local boss, find some treasure, then come back up without ever investigating that trap door underneath the sarcophagus--which is what will usually happen.
Here's a secret--this is my game's Big Diagram--it shows how various dungeons and dungeon entrances are connected. You have to imagine the big grey dots are ground-level dungeon entrances and the pentagram is at the center of the planet.
|Click to enlarge|