See, I'd like a post-apocalyptic fantasy setting that started out as generic fantasy, and ended up some kind of weird alternative fantasy. Admittedly, that's kind of what Dark Sun is, but the focus there isn't usually on how the world has changed so much as that the way the world is and how tough it is to survive. I'd like something where a world more or less like the Forgotten Realms (although probably all of one race) goes through some big, world-changing catalysm--like a planar conjunction made worse by magical meddling, which then allows invasions from multiple other dimensions, screws up the fundamental nature of magic in a few ways, and introduces powerful and dangerous artifacts and materials from alien worlds. The resulting setting would be a broken world that remembers (probably through very rose-tinted accounts) its past glories, that's now full of weird crap that's alternately terrible and wonderful, and it slowly putting itself back together in some strange new shape (the exact nature of which is subject of much conflict). Semi-dark, but optimistic.