Akavir! AKAVIR! AAAKAAAAVIIIIIR!
Nah, it'll probably be Summerset Isles or Valenwood... maybe Elseweyer. Most likely, I'm betting it'll be the entirety of the Aldmeri Dominion- a bit difficult to manage, the combined nations would probably make them Morrowind-sized individually (as in, Vvardenfell, not the whole thing), most likely, and would probably require two radically different general architectural styles. Granted, that's nothing gamesas hasn't done before; compare Ebonheart and Caldera in Morrowind with, say Vivec, Ald-Ruhn, or Sadrith Mora. And in Oblivion... pick a city, compare them with the others, or with the Oblivion worlds. Same with Skyrim... you have several sections of the game that are radically different from the main map. Not counting DLC content, just stuff that was in the base game.
If this were true, I'd imagine that the game would start in Valenwood, and you'd make your way to Summerset Isle the closer you were to the endgame (going, of course, by the Main Quest.) There might be a Psijic Order sublot too, if not them being the driving force now that the Empire seems to hold limited sway. Remember- Skyrim is the first game where the first part of the main quest wasn't given from someone in Empire proper. That is a significant change in the equation, if it hasn't been pointed out already. The Empire's role, should it succeed, will likely be of a background force in future games, sometimes helping or hindering the player, rather than be an official "ally."