I voted Ravenloft, Forgotten Realms, Planescape, Dark Sun, Spelljammer, Ghost Walk.
Magic: The Gathering
I'd really enjoy seeing a blending of Magic the Gathering and D&D. That would actually make me consider buying all the new core books. Have Magic the Gathering as a setting, with races and monsters based on hte cards, and spells that I love in that game. Effectively, a sanctioned version of the Dusk setting that I can't buy because they shot it down. Why they haven't released a Magic(tm) setting in d20 already baffles me, it would attract magic players to D&D and increase their maket share. I've made elves from the new set as a ecl 0 race for example. And with how often they release magic sets, they could keep seeling books for the settings and spells and creatures and we wouldnt have to change rulesets every couple years! I supposed this would make Magic the Gathering multiple settings, or a massive setting with a huge cosmology and multiple prime material planes. They would have to retrofit to include things back to about 5th edition for it to be worthwhile.