For those that mentioned a d20 Dark Sun, you might take a gander at Athas.org. Good stuff there. There was a treatment in Dragon/Dungeon a while back, but that's had mixed reviews.
For Shadowrun d20, there is a netbook out there somewhere that adapts Shadowrun to d20 Modern as well as another site that has done some work.
I'd like to see d20 Spelljammer as well, done in a hardback and with lots of giff.
I'd also like to see d20 versions of Mystara, Al-Qadim, Maztica, and Star Frontiers. I wouldn't mind a d20 Planescape if the artwork was different (more like what was in the campaign classics article in Dragon). Give me a d20 version of Tale of the Comet as well.
And let me jump on the bandwagon for an Expanded Oriental Adventures, with Kara-Tur as the default setting! Not that I prefer Kara-Turn to Rokugan, just that I'd like to see a sample setting that presents as many facets of OA as possible.
More psionics support from Malhavoc. I also wouldn't mind seeing Bruce Cordell give a shot at a setting where psionics are a major factor in the world.
Epic-level Bloodstone sourcebook for the Forgotten Realms.
Age of Worms compilation. (I'm betting this is just a matter of time.)
All I can think of for now.