While I know that Greyhawk and the Forgotten Realms have the most supporters, I'd like to see "Dungeons & Dragons" Fourth Edition support (by third parties, if need be) for Mystara, Dragonlance, Al Qadim, and Kara Tur. This may indicate some nostalgia for OD&D and AD&D, but there has also been little support for many of these in a long time.
Ravenloft/Masque of the Red Death, Dark Sun, Council of Wyrms, and Spelljammer would be close behind for me. That last one might be easier to do with the new cosmology. I'd also like to see D&D4e worldbooks for historical settings, similar to the AD&D2 and D20 books for Rome, Greece, Egypt, Vikings, etc.
Of course, with the revised OGL, nonfantasy worlds that are compatible with D&D4e will be scarce, and I prefer the D20 references for modern and science fiction settings that didn't use D20 Modern to those that did, but that's probably a discussion for another thread.