Oh I think it'll definitely be Elsweyr. Bethesda aren't gonna try to redo one of the old locations, and I think that the whole of Tamriel just isn't them. My views on the remaining are:
Summerset Isles - Too monoculturistic. In fairness, nobody wants to spend 200 hours on a game only looking at a bunch of snooty Altmer trying to become a master race. There wouldn't be enough opportunity to even see other races there, let alone play them. I think I'd die of hatred playing an Aldmeri Dominion based game as well!
Black Marsh - Again, the harsh environment of the marsh excludes many races except the Argonians. I think there's nore likely to be a TES VI: Black Marsh than Sum Isles, because you vould more easily fit the other races in - the Argonians don't hate everyone. I think it might be a bit Morrowind-esque too..
Valenwood - Perhaps, perhaps. Not much diversity there though, I can see people disliking the extensive forests. Hey, who's gonna convince a Notherly Nord to stick around down' south with the Elves? I think this is a possibility, and I wouldn't mind it, but I don't think we know enough about the Bosmer to want to go there yet. I can't really see an opportunity for the main quest either.
Orsinium - Ch'yeeaah. This place was actually destroyed (twice), most recently around the Early Fourth Era by the Redguards. Maybe, if there was a decent time gap between Skyrim and Orsinium, but I doubt it. It just doesn't seem right Bethesda doing an Orc-based game.
Elsweyr - Boom. This is where its at. There are deserts, jungles and a coastline. There's also lots of opportunity for a main quest. Skooma trade, trouble with Valenwood etc. Also, Khajiit are generally preferred than Argonians. I really think that Elsweyr is going to be TES VI.
Forward, to TES VI: ELSWEYR!
~daT.