I'd rather High Rock, the new Orsinium (not included as Hammerfell) or Atmora as place for an expansion for Skyrim as they are better suited and have a lot less chance of being a likely candidate for a future game setting in the series. High Roch has little chance as it'll be too much like Skyrim and people would want something different.
High Rock was featured in TES II: Daggerfall, as was part of Hammerfell. Hammerfell was also featured in "TES: Redguard".
And the Empire and the Dominion both want a piece. Other than Alinor Hammerfell has the most story potential as of right now.
The Aldmeri Dominion isn't exactly interested in Hammerfell itself, they just want it so they can freely pass through it to attack the Adamantite Tower on the Isle of Balfiera.
I think Bethesda could take a leaf out of Ubisoft's book and release games a bit more often. Not every year like AC but every two maybe. They have the Lore, so it's just about coming up with a decent story.
Don't even mention Ubisoft, especially with what they did to the Might and Magic series. They basically took all the lore of the previous M&Ms, and threw it out the window. Liches are no longer the result of spell casters using a necromantic ritual to prolong their lifespan at the cost of their mortal body, but are now priests blessed by some spider goddess *cough*LolthRipoff*cough*. Vampires are no longer the result of a necromantic curse that spreads through bite, but are former Liches that were granted an improved body by their spider goddess. Wtf?