According to http://bquick.info/wp-content/uploads/2011/02/TamrielMap.jpg map, Summerset Isle seems too big for an expansion and I think that it should have game of it's own or next TES could include Summerset Isle, Valvenwood and Elsweyr at once since they are not that big provinces and I think that technology would allow them to make it that way.
TES after that could ignore Black Marsh and they could go for TES VII: Tamriel (I have right to dream -_-).
Still, even if ti wasn't too big for an expansion, Bethesda doesn't make expansions anymore if I'm not wrong, but will make only multiple DLC's with smaller chunks (there is an article about it and reasons behind it, but I'm to lazy to search for it
).
Also, if we could see a part of Tamriel in a expansion set, I would rather take a look at rest of the Morrowind.
With all this war going on there and Argonians owning them, I think there is potential for a good story that could connect with Skyrim one.