Man, I am of several different minds on this question.
On one hand, my initial urge is to echo Asmor and say that if they're going to bother to resurrect a setting we've already seen before, then let it be Planescape, Dark Sun, Al'Qadim, or Spelljammer. Greyhawk by its very nature doesn't need to be a published setting, because it's basically just generic D&D. Even Ghendar admits that Greyhawk's strength is that it isn't strongly defined, allowing individual DMs to make it their own. (If anyone knows anything else good about Greyhawk, please fill me in. Because I am seriously drawing a blank, here.)
Following Ghendar's point, I really have to admit that I don't really like using other people's settings, anyway, and I'd rather just have more cruchy bits I can string my own fluff on. More Tomes of Magic, more Books of Nine Swords, more Unearthed Arcanas.
But at the same time, I do know that new campaign settings always bring along lots of awesome new ideas to play with. So in that light, yeah, I'd most like to see a bunch of one-off settings with no heavy support. And, for God's sake, let them be settings that don't have a place for every damn monster, race, spell, and magical item that's ever been published, unlike Eberron. I think a setting's flavor can be defined as much by what isn't there as by what is. Also, I kind of dig the idea of letting these one-offs be very artist-lead ventures. What if they let Brom or Tim Bradstreet or Mike Mignola or Wayne Douglas Barlowe or, hell, some teenaged prodigy from DeviantArt--anybody with a unique and consistent visual ethic--just go to town, working up whatever kind of world their own preoccupations lead them to, and then letting the regular developers flesh it out and stat it up afterwards?
And, please, no more settings with blatantly artificial character-class-based social structures, elemental magic systems, and law-vs.-chaos conflicts built into them. Seriously.