1) some kind of quasi-medieval world, like Greyhawk or FR.
2) some kind of steampunk world like Eberron
3) some kind of post-Apocalyptic world like DarkSun
4) and Spelljammer to bind them
Plus some kind of detailed multiplanar structure to adventure in. While I'm a big fan of the Pre-4Ed cosmology, I could see doing something else, if it were done right. Perhaps a vertical cosmology, like found in Clive Barker's Imagica.
2) some kind of steampunk world like Eberron
3) some kind of post-Apocalyptic world like DarkSun
4) and Spelljammer to bind them
Plus some kind of detailed multiplanar structure to adventure in. While I'm a big fan of the Pre-4Ed cosmology, I could see doing something else, if it were done right. Perhaps a vertical cosmology, like found in Clive Barker's Imagica.