I've got to throw in with those saying Summerset, and a storyline centered around the Thalmor. Not that I wouldn't also like to go to Elsewyr or Blackmarsh - I think both of those lands could have some truly awesome story telling potential. I just think that, having introduced the Thalmor in Skyrim, and the Psijics (ok, ok, I know the Psijics have been in the lore forever, but so far as I know, Skyrim's the first time you can actually interact with them, sort of).
I'd like to go to Summerset to try to overthrow the Thalmor, and to learn more about the Psijics and Arteum. Also, find out what became of the Eye of Magus, and what it does.
Finally, with Skyrim being a game where, because of the setting, magic is downplayed, it'd be cool to swing the pendulum to the other side and go to the most magical province, and have a much more magic-focused game.