Nyambe.
Northern Crown.
Oathbound.
Testament.
Dark Sun.
Planescape.
DragonStar.
I'd
loathe for the 'mythic culture' books (Maztica/"Southern Crown", Kara-Tur/"Oriental Adventures", Al-Quadim/"Arabian Adventures") to be linked to FR. I don't like FR. I don't want to play in FR. I don't want to even entertain the thought that someone like Elminster is even in the same game.
Take the Nyambe route, take the Rokugan route, make them self-contained little places you can either use on their own, or drop into other settings as needed.
This is also my issue with Spelljammer. Other than the fact that I think DragonStar did "Fantasy in Space" much better, without the inherent absurdities and preponderance of anthropomorphic animal-things, SJ has a few good ideas. Elven and drow armadas, Illithid space-vessels, Beholders the size of planets...that's cool.
Planescape I somewhat except from the 'connection to other settings' thing because Sigil's whole motif is that it connects to everything, anywhere, ever. Inifinite possibility is part and parcel of PS's appeal, and it needs to be preserved.
DarkSun is cool, no questions.
Oathbound deserves a 4e book. That setting kicked hard.
I would like a series of 'genre', and 'mythic world' style books, so we could get supplements for various locations and ages, all self-contained.
About the only settings that don't fit into those models are Dark Sun, Planescape, and Oathbound. The other ones I'd like to see done are all "D&D versions of Place X" or "D&D versions of Genre Y" Which are cool, but aren't really unique settings.
As a postscript, I do want rules to play dragon PC's in 4e. One of the coolest one-off adventures I played in 3e was after the Draconomicon came out and people rolled up dragon characters. It was AWESOME.