As you already know, I'm kicking around the idea of creating my own megadungeon. For the last couple of weeks I've been toying around with different approaches of the basic setup, i.e. what the story of the first two or three levels should be. I've thought about an abandoned dwarven city, or an abandoned dwarven mine (I really like dwarves). Today, still with the impressions of this thread in mind (and several posts of Evreaux, too), a lever in my brain must have been pushed. Why not a totally otherworldly dungeon? I mean, it is the freakin' Mythic Underworld!
Here's my setup:
Deep beneath the earth, the powers of Chaos reign supreme. Where the surface world belongs to the immortal gods, the realm below is ruled by the primordials. Beings of uncomprehensible power, older than time itself, alien to mankind (if you think "great old ones" you're on to something). At least that's one theory. Anyhow, as a matter of fact, the primordials and the immortal gods alike are really bored, their eternal struggle recessed because of a recent truce (only a couple million years old), so they arranged a game.
The gods would choose some mortal champions and the primordials would provide the playground. The chosen heroes are sent into THE DUNGEON; gods and primordials bet on how far the groups will make it. The game is about mortal souls.
With this outline set, the gods implanted the urge to adventure into selected individuals at birth. Driven by this urge for their whole life, these people somehow eventually make it to the small, totally unimpressive, rural town of TANNENHEIM, where they soon hear rumors of a stone circle. Who spends a night sleeping in this circle may have strange dreams and may also find the entrance to a strange cave system of unknown depth, where great riches are just waiting for bold adventurers to take. Enter the new heroes.