Definitely the Aldmeri Dominion. The whole Thalmor thing in Skyrim was never really resolved although it was a pretty major part of the game's story. Also, the only locations that haven't been the focus of their own game (as far as I can remember, it's been a while lol) are Summerset Isle, Valenwood, and Elsweyr. Two of those provinces are elf-based, so there's a good chance that the next game will be themed around elves, and, consequently, the Dominion.