My two cents on the matter... WoTC has two major things that it's going to take its cues from when it comes to reviving settings; preserving intellectual copyright, and popularity with the fanbase. They have, after all, explicitly stated that the "many settings" approach was actually a financial mismanagement. So, here's my opinions on the settings that I'm aware of:
Planescape: This is perhaps the most iconic "weird fantasy" D&D setting. It's also getting a lot of promotions between things like Mordenkainen's Tome of Foes and the recent web-serial "Hell's Belles", which is about a D&D group playing a 5e translation of a Planescape adventure called "The Great Modron March". Properly handled, Planescape is nothing like the Forgotten Realms, and whilst they're most likely to make some major tweaks to the setting, it's still looking like the best and most likely choice for a new setting.
Nentir Vale: They literally had a setting book for the "Points of Light" world ready to be printed under 4e before they hit the cancel button on that, and it does offer a very different and yet still familiar approach to the formula. I wouldn't say this has the highest chance, but I want to think it has a better than average chance.
Greyhawk: This one I honestly think has the lowest odds of getting in as, on the surface, it's too close to the Forgotten Realms. Both have a very strong "Neo-Medieval Europe" feel, and whilst Greyhawk is more Sword & Sandals than the Realms are, that difference isn't enough to make it seem like a "good sell" from WoTC's perspective. Indeed, Greyhawk's comparatively low-fantasy stance is actually an argument against its revival, simply because that minimizes the usefuless of other splatbooks - Greyhawk is supposed to be a world where only the "Neo-Tolkien" races are abundant, whilst the Realms were the first setting outside of Dark Sun to have an official thri-kreen population.
Spelljammer: I don't see this as very likely, simply because the execution was extremely goofball focused. A "5e Spelljammer" would probably be more of an in-name-only translation, and its more likely it'll be folded into Planescape than anything.
Birthright/Dragonlance: Both of these are falling into the same position of "kind of Realmsian, but they do have a unique hook". They're more likely than Greyhawk, but still, I don't think that these would be WoTC's first choice.
Mystara: Again, this "Pulp Fantasy" setting isn't that close to the Forgotten Realms when you look close, but it can seem similar at a casual glance. Also, the fact it's such a mash-up of expies of various real-world cultures and peoples alongside its more fantastical races is problematic in this day and age. With "cultural appropriation" such a media hotspot, as much as I miss the lupins, aranea, diaboli, enduks, scorpionmen, phanatons and so forth, I don't think this setting is likely.
Al-Qadim/Kara-tur/Maztica: They're all officially part of the Realms and, even more so than Mystara, they are a big PR risk from the "cultural appropriation" crowd. As much as I want an official Oriental Adventures splatbook for 5e, I don't think any of these are going to make it into the game.
Ravenloft: I think we can cross this off the board altogether; the Ravenloft setting is obscure, the last version was handled by a 3rd party, and WoTC already refreshed their IP rights to this by publishing Curse of Strahd.
Eberron: The most popular fantasy setting of recent years, it was well-received in 3rd edition and 4th; it's got the biggest existant fandom of any of the settings I've named here and it requires the least amount of new rules - yes, yes, I know Kalashtar need psionics, but Kalashtar aren't from the Five Nations, so they can easily delay them until they do a 5e equivalent to "Secrets of Sarlona". Whilst coming out in 4e means that IP isn't a big priority for them, still, this is the big, obvious market to tap.
Dark Sun: This got a major shot in the arm with its well-received 4e release, but because of how much work it needs in order to be pulled off right, and how little a concern its IP is, I would put money against it being released this year.
In conclusion, I think we can be assured of seeing Planescape and either Eberron or Nentir Vale/Nerath/PoLand this year. My money is on Planescape and Nerath, simply because the Nentir Vale requires the least amount of work from WoTC to make ready.
