Kara-Tur. I'd like to see it spiffed up a little and maybe the timeline moved about 20 years from when it was created. Doesn't have to be multiple books, just one like the Eberron Campaign Setting with a smattering of info for players and DMs.
I really liked Mystara. Back before it had that name, really. It just had a nice chivalric romance feel to it, along with all that swords-and-sorcery stuff.
Can anyone even name all the settings which Wizards of the Coast owns?
Top Secret S.I.
Gamma World
Dark*Matter
Magic: The Gathering (perhaps by specific setting like Ravnica: City of Guilds) Dreamblade
Dragonlance
Star Frontiers
Boot Hill
And what about all the settings owned by Hasbro...
Greyhawk is the only one I want to see brought back with regular releases.
Birthright is the one I want to see most, but I would rather it be limited support. A core book and maybe one or two supplements supporting it. Maybe periodic releases could be done through D&D Insider. The country books could easily be released that way. Just reprint the old fluff and change the limited number of rules elements (stat blocks, etc).
There are only two I don't want to see at all. That's because they are "metasettings" that try to force themselves on too many things. The rest can be released as single books on a periodic basis.
I voted for Dark Sun, Planescape, and Spelljammer, but what about all the other's?
What about an unholy mesh of Alternity and D20 Modern for a new non-D&D fantasy ruleset? This would be a great place for things like: Boot Hill, Dark Matter, Star Drive, Gamma World, Top Secret, etc.
I don't know several of those settings, but of those I do, they're all pretty much the same.
Greyhawk: Standard Fantasy
Maztica: Standard Fantasy in a Central American Wrapper
Kara-Tur: Standard Fantasy in an Oriental Wrapper
Greyhawk: Standard Fantasy in a Horror Wrapper
etc.
I voted Dark Sun (which just barely breaks far enough from the standard fantasy paradigm for my tastes), Spelljammer, Planescape and Al-Qadim (admittedly, Al-Qadim is just Standard Fantasy in an Arabian Nights Wrapper, but what can I say... it appeals to me ).
Frankly, even having Eberron and Forgotten Realms being published at the same time seems superfluous to me.