I think Hammerfell is the most likely location for the next Elder Scrolls game, when you consider the current state of each province.
Cyrodiil: Not likely. The last Elder scrolls game took place here, not many people would want to go here again so soon.
Black Marsh: Not likely. Not much going on here overall. Argonians invading southern Morrowind. A marsh also doesn't sound like an interesting setting for a game, but that's just my opinion.
Elsweyr: Possible. Currently a client state of the Aldmeri Dominion. Maybe the Empire will gather enough strength to take the province back.
High Rock: Not likely. Not much going on here.
Morrowind: Not likely. Vvardenfell was destroyed and southern Morrowind is being invaded by Argonians. A lot of Dark Elves have fled to other provinces.
Skyrim: Not likely. They wouldn't make the next game in the series take place in the same province.
Summerset Isle: Not likely. You'd have to be a member of the Aldmeri Dominion to have any reason to be there.
Valenwood: Not likely. Part of the Aldmeri Dominion. Even if the Empire was invading, it is unlikely that the player would be a part of that invasion, because that wouldn't allow for much variety in quests.
Hammerfell or Hammerfell/High Rock: Likely. Hammerfell is currently independant and has fought off the Aldmeri Dominion. The Aldmeri dominion will most likely invade again. If the Empire decides to fight back at the Aldmeri Dominion then Hammerfell would be a very useful ally. Hammerfell seems to have the most problems right now.
Of course each game seems to have a large, unexpected event that takes place after the previous game, so it could be anywhere. It wouldn't make much sense to have the whole Aldmeri Dominion thing fade out of existence without the player being involved though.
Maybe a large, unrelated event will take place in Hammerfell or High Rock with the Aldmeri Domion being a part of that story, but not directly related.