Personally, I've always wanted to see Elsweyr. The jungles would be tough on PCs, but we could see some really interesting weather effects, environments, and enemies. Stealth gameplay and trading/barter would be revamped since they play such a large role in Khajiit society. Quest writing could really be improved. You'd never know what twists and turns a seemingly 'helpful' Khajiit could lead you through, since they tend to be manipulative and self-centered.
Realistically, my bet's on High rock or Valenwood though

As much as I'm enjoying Skyrim, it would have been my last choice, except possibly High Rock.
Edit: Black Marsh is waaaay in the lead. I'm surprised Elsweyr is so far back
