Been Gone a While

It would be a start-up adventure for Planescape.

Something about the idea of players in a normal prime world thinking that they're busy destroying this all-important all-evil mage, with the slow creeping realization that the dungeon they entered (through an ancient, decaying portal) was apparently engineered for adventuring parties (helpful hints along the walls, sporatically placed rest-sites with fully stocked mini-bars). The document where they read about the mage turns out to have been a half-burnt ad that ended up in their prime-world; they get introduced to Planescape by discovering that for the rest of the Multiverse, what they dedicate their lives to (dungeon-romping and evil-slaying) is pretty much just a form of recreation for the bulk of the population.

One of the big themes of Planescape I've always enjoyed is the contrast to the Prime. In the Prime, you matter. The things you do--the villains you beat--the dungeons you explore, the monsters you slay--they can reshape the world.

But in Planescape, no matter who or what you are, you're just a berk. Even level 20 characters are just ants in the eyes of the real powers. The multiverse is just too immense for you to have any truly lasting effect.

Plus, the wizard that's used as the villain is a half-mad loony who's been ressurected so many times that he's actually trying to get perma-killed.

In addition, I have a crazy idea about a Yugoloth-run Library of Lies (where the Loths forge entire histories of lies, perpetuating untruths across the Multiverse--including false creation stories for the Baatezu, Tanar'ri, and anyone else) where the deceit gets so deep that it sometimes wraps back around into truth, and the lies themselves become alive. Even the Yugoloths in the library itself are victims of their own lies--and somewhere in the library is said to be the most powerful lie of them all: the First Lie.

A Mechanus math-golem (as in made of numbers--as in, he has no physical form, but instead insinuates himself into systems). A bard who's stories are so persuasive that they take on a life of their own and proceed to romp through the Multiverse (mazed after he started telling a story about the Lady of Pain). A haunted asylum in Sigil run by a brilliant disbarred Bleaker obsessed with immortality through memory, who terrified the inmates until their memories of him were so filled with fear that their collective unconsciousness allowed him to survive as a terrifying nightmare long after his death. A plane that's literally a tangled knot of space and time, where portals lead everywhere--and every when. A mage of time who's crimes were so great that he was trapped in a miniature demiplane that travels backwards through time.

Tons of other stuff. My brain is bursting.