I still think it's going to be the Abacean sea!
Let me be more specific: It will (I desperately hope) include the southern coasts of Hammerfell, the Gold Coast of Cyrodiil (including the Thalmor-occupied city of Anvil), the Northern coasts of Valenwood, and all of the islands in between. They may include the island of Moridunun, but I highly doubt they will unveil an updated vision of the Summerset Isles until The Elder Scrolls VII, when they've had time to optimize their use of the next-gen consoles.
Choosing a world map that allows for an emphasis on naval battle and exploration would be a refreshing new gameplay dynamic, so that the gameplay doesn't come off as just an "upgraded Skyrim". It would also allow players to explore the cultures of various provinces, which would be engaging for dedicated solo TES gamers as well as (perhaps more incidentally) the MMO players who may get drawn into the official series by TESO, because for them it wouldn't feel limiting after having been able to travel all across Tamriel in TESO. That's not to say I think that Bethesda will take seriously what MMO players think about the main series, I believe the feedback from solo gamers will continue to be given a priority in terms of game development, as it always has.
As for the time frame, I want to say it's likely that they won't take it too many years after the events of Skyrim. I'm predicting a quick succession in terms of in-game time, much like how Morrowind and Oblivion took place very close together on the 3rd era timeline. The Abacean is far enough from Skyrim, and busy enough with the Redguards restisting the Thalmor, that news from Skyrim won't be arriving very frequently. This bipasses having to worry about addressing the winning faction of the civil war.
As someone in the last version of this thread responded to this idea for a setting (that was the person above me, actually), the game will probably be heavily focused on redeeming the Altmer, by (first) having the Thalmor fail and show weakness in some major way, and by showing the grace, power, and honor of the rebel Altmer. I love that idea, and my take on that is that the more esoteric aspect of the plot (there always is one, thanks in large part ot MIchael Kirkbride I think) will be further reinforcing the more hidden prerogatives of the Thalmor agenda, as hinted at by Ancano's attempt to harness the Eye of Magnus, and said Kirkbride chap's "Altmeri Commentaries on Talos", which extrapolates on the plot to reclaim the High Elves' Ehlnofey status by destroying man in order to follow in Auri-El's wake. A huge part of me sympathizes with the ultimate goals of the Thalmor... but I believe that the events surrounding Magnus in the College of Winterhold storyline, and the intervention of the Psijic Order, suggest an attempt by the Thalmor to tap into more powerful magics by claiming other artifacts through military aggression, and the struggle that the resistors and an increasingly active Psijic element will face in stopping them.
The Abacean would be a choice setting, introducing a myriad of colorful environmental themes, which are a much-needed thing after facing the bleakness of Skyrim. And, like I said, aren't we sick of our characters not being able to sail so much as a rowboat???