Anyone else been reading Girl Genius lately? I've been thinking that Castle Heterodyne would make for a pretty good megadungeon. In particular, the idea of the place itself as sentient and semi-mobile appeals to me, since it gives me a great excuse to change things up and add in new areas as needed. The details of the castle's personality are pretty good, too -- mechanical malevolence is as good a reason as any to have tons of death traps littered about the place, and the fractured nature of its mind means that even if you make friends with one part of the place, the rest is still out to kill you.
Anyway, the upshot is that I'm smashing this idea into my "physical manifestation of the netherworld" idea to get the general outline of the dungeon I've started. Essentially my idea is that the place was built as a fortress by a family of Mad Scientists (or possibly, y'know, wizards) but that over the years, said family's vile and debauched behavior built up a malign aura about the place. Over time, the already-bizarre complex became intertwined with the Netherworld . . . and perhaps other, much darker places.
What this means in actual practice, I'm not quite sure yet. I know that at least a few sublevels will be tombs, possibly with surface works attached. (And by "are going to be" I mean, "unless I get a better idea/distracted." At the very least, I'll have "The Tomb of Zagyg," full of death traps and weird random effects.) The initial first level -- what I'm working on right now -- is The Labyrinth, originally intended as a defense system for the fortress. (And amusement for its owners.) And at some point I'm going to have a section that is very much The Netherworld, all skulls and ghosts and stuff. But beyond that, I don't have any specific plans. I figure I'll kick down that door when I come to it.