Birthright, because it's my favorite setting that TSR/Wizards has ever published for D&D. I know it doesn't have a snowball's chance in hell of being done as a 4e book, but I would love to see
Dragonlance, because it's a classic D&D setting, and many parts of the setting would need game mechanical support, like stats for PC minotaurs, moon magic and the wizards of high sorcery, and the knightly orders. I'd love to run a Taladas game, in a minotaur league reimaged as an emerging empire amid points of light.
I didn't vote for Kara Tur, because it's just a relatively dull conversion of the historical far east with the serial numbers filed off. I would love to see an oriental advensures sourcebook though.
Eberron, if only because I'm playing in a couple eberron games right now, and it has many world elements that would need new game mechanics.