Glyfair
Explorer
Masquerade said:QFT. Hate to say it, but, if Eberron is any indication, there are still interesting new settings out there that D&D has yet to explore. I would much rather see something original, especially given that the fluff from these old settings is still usable.
The reason I choose Birthright is because, IMO, it's the only one I want to see that doesn't work in 3E. The bloodlines experiments done so far can be combined into one that ties into Birthright fluff.
In addition a domain management system, and a realms spell system haven't really been touched by WotC (I know a few 3rd parties have touched on domain-like systems). Both are assets.
The only other setting that needs new "rules" is Spelljammer (don't know enough about Maztica) and I don't want to see that setting revived (unless it is completely self-contained).