Valenwood or Elsweyr, or better yet Valenwood and Elsweyr, call it Elder Scrolls VI: Dominion and have see the fall of the Thalmor(and probably some world threatening danger you need a hero to solve).
I think there is a lot of interesting stuff that could be done with having two provinces in one game, especially when both of those Provinces are already a bit smaller than some and come with a lot of historical baggage between the Wood Elves and the Khajiit.
As cool as I(and probably many others) think it would be to do just Elsweyr or Black March, the cynic in me it telling me I don't think we will see a Elder Scrolls game featuring mostly Cat or Lizard people. Perhaps my concern is unfounded, but in this case I have an image of focus groups talking about how X% of people don't want to play a game about those races(even though the other races would almost certainly be playble.)