Dire Bare
Legend
Uniting the community - if that's one of the goals to one degree or another - is yet another great reason to use Greyhawk. Everyone, regardless of which edition they support or play the most knows of, has played in or in some way interacted with Greyhawk over the years. There are whole websites dedicated to it today supporting the lore (both pre- and post-Wars and every time in between), the sandbox is big enough for everyone. Plus there's been efforts in some corners of the Interwebs to go beyond the continent of Oerik.
Now don't get me wrong, suggesting Greyhawk is in no way meant to denigrate the other worlds. I think the grognard community at large would welcome the return to Greyhawk as well as the number of newer players that would come to the community to continue the growing setting.
I love Greyhawk myself, but whenever it comes up, a fair number of people diss it.
I assume Forgotten Realms is the most popular setting, but I don't know for sure. I have no personal interest in/very little knowledge of FR, but my impression is it's the leading brand.
I don't think we really need to get WotC to release their secret sales numbers on this one. It's pretty darned obvious that the Realms is the most popular D&D setting ever . . . . if it weren't, those classic Black Isle video games would have been "The Free City" instead of "Baldur's Gate", we'd have a huge catalog of Greyhawk novels, and we'd be seeing "Gary Gygax's Greyhawk" this fall instead of "Ed Greenwood's Forgotten Realms".
I love Greyhawk (and really, all the classic settings), but it isn't really the best choice for the "default" setting. WotC already tried that with 3E, and it didn't seem to work all that well for them. Hardcore gamers are familiar with Greyhawk and hold it up as Gary's holy work, but most gamers are much more familiar with the Realms. Not a diss on Greyhawk or even a plug for the Realms, it just is what it is.
If your goal is to produce a "unifying" edition, you already know that even if you succeed beyond your wildest dreams, you will leave some fans out in the cold. Why would you choose an older setting that relatively few are famiilar with and love, rather than an only slightly newer setting that has dominated D&D and its fandom for decades?
As we discuss the issue, I'm coming around to this scenario as my personal favorite . . . . use FR as the default setting, but give it a light treatment in the core books, much like Greyhawk got in 3E and the new "Nerath" setting got in 4E. Revamp the setting to try and keep what was cool from the 4E cosmology, but bring it back to a more classic feel (subjective, I know). Don't bother with yet another RSE (Realms Shaking Event), just reboot and not worry about breaks in continuity. New FR-branded books (and DDI support) can deliver the immersive world-building that FR is know for.
Make sure that the other existing settings get a good page or two in the 5E DMG, to both celebrate the history of D&D and to show new DM's how different D&D can be with some tweaks and good story.
Then, for other settings, from the classics Greyhawk and Mystara to the more offbeat like Dark Sun and Eberron, give us one beautiful, coffee-table, big-ass celebration of a book for each setting, and leave it at that . . . except for more support in DDI as demand, well, demands.
Then give us another Eberron, something brand-new and different. But make it a limited run of perhaps a dozen books or less.