There was some rumors about it being Valenwood and the Summerset Isles. I seriously doubt they will repeat, which means no skyrim, morrowind, cyrodil, or high rock. Given that they will most likely keep the time frame linear, so post skyrim, it will most likely revolve around the aldmeri dominion.
As an interesting side note, todd howard has stated that there are two big projects in the works before ES VI, and a part of me thinks they may do an rpg set in akavir.