First off, I agree with [MENTION=1210]the Jester[/MENTION] that WotC is more likely to do one a year rather than two.
While I like the idea (obviously, considering I brought it forward!), I can see how, once you get beyond a few settings, there are good reasons not to do a full treatment. Let's take a look, with some random thoughts on each:
Forgotten Realms - they've already said this is the default/core setting for the game so its a no-brainer.
Greyhawk - given its status as the classic D&D setting and 5e's design goal of being Everyman's D&D, this would also seem to be a no-brainer to revive. Also, as I said in the OP, its never really gotten a full treatment, at least not for decades.
Eberron - I have a feeling that Eberron might be done. It seems moderately popular but maybe not enough to create another book.
Planescape - This is a big question mark. On one hand, I could see them doing what they did in 4e - include Sigil in the Manual of the Planes. But they barely skimmed the surface. I think a full treatment of Sigil (with sections on the Outlands and planes) in hardcover would be quite successful, sort of WotC's answer to Ptolus.
Dark Sun - Dark Sun, while specific thematically, has a place. I think it deserves another treatment and has the following, or at least "collector appreciation," to be viable.
Dragonlance - I think Krynn's day is done. While the Realms can be relatively painlessly rebooted as it was always an RPG setting first, a setting for novels second, Krynn seems a bit more towards the latter, as the number of people who read the novels far outweighs those who actually played Dragonlance. The metaplot is too dominant. I suppose the only way to do this would be to either move the timeline forward again, to a relatively clean slate, or reboot back to Chronicles, and come out with Paizo Runelords-style adventure path book in addition or instead of the campaign setting.
Mystara - I always think of this as Greyhawk's younger sibling. I remember buying a couple products just for the Stephen Fabian art, but being turned off by bearded elves. Anyhow, I don't know what to think of Mystara. On one hand, I think the nostalgia factor would, like Greyhawk, give a guaranteed number of sales. But would it be enough? Is Mystara not quite classic enough to avoid the dated feeling?
Ravenloft - I believe this had already been announced as being part of 5e canon, so we'll see what they do with it.
Nerath/Nentir Vale - This setting seems woefully neglected. I'm of two minds, either let it go as an artifact of 4e, or go whole-hog in and create a setting out of it. Its a tricky one.
Birthright - While I liked Birthright, I can't see a reboot. Maybe if they wanted to do something along the lines of Paizo's Ultimate Campaign and Kingmaker AP - Birthright would be a good default setting for that. But it just doesn't seem to have the gravitas to stand on its own or be worth the resources required.
Spelljammer - Spelljammer is sort of like the disco of D&D worlds - its better in memory, and in short doses, than in reality, and larger immersions. I enjoyed Spelljammer, especially space warfare, but it seems best suited incorporated into something else (which is why I loved the 4e Astral Sea Planescape-Spelljammer fusion).
Al-Qadim, Maztica, Kara-Tur, Hollow World - I can't see any of these being serious options, at least not on their own.
In summary, I think we could split them into the following groups:
Tier One (Probable - keep the flame alive, WotC): Forgotten Realms, Greyhawk, Planescape, Dark Sun
Tier Two (A good possibility, but not definite): Eberron, Dragonlance, Mystara, Ravenloft, Nerath
Tier Three (Probably best left in the closet, or incorporated somewhere else): Spelljammer, Birthright, etc
With that in mind, I would do full treatments of tier one, with a three year plan like so:
2014: Forgotten Realms
2015: Greyhawk, Planescape
2016: Dark Sun
By 2015 I'd be thinking about 2017 and beyond. I'd consider how the first few went and what community demand is and then maybe start dipping into tier two and/or create a new setting.
2014 will be full enough, so I think the Forgotten Realms is enough. But I think Greyhawk and Planescape can be done in the same year, because they're both related and different enough. Planescape could be considered an extension of whatever kind of Manual of the Planes book they do, and I believe MotP has traditionally been released (at least in 3e and 4e) within a year of the core rulebooks.