I personally think the next Elder Scrolls game will take place in the Summerset Isles, and here's why:
Most TES games have taken place in the Northernmost half of Tamriel, with none of the main series games (save for Arena) having you ever set foot in the four southern provinces of Tamriel: Black Marsh, Valenwood, Elswer, and Summerset. And with all the chaos caused by the Third Aldmeri Dominion A.K.A. The Thalmor in the last two Elder Scrolls games, it seem inevitable that they will be an integral part of TES VI, therefore it seems only logical to have TES VI take place in the Altmer's homeland, as well as the headquarters of The Thalmor.
It would be nice to see an Elder Scrolls game with all of Tamriel accessable though. What do you guys think?