Think of this. How big was Oblivion? http://www.uesp.net/wiki/File:Cyrodiil_map_Oblivion.jpg
How big is Skyrim? http://images.uesp.net//3/33/Skyrim_map_Oblivion.jpg
You can really see it here. http://images.uesp.net//c/c3/TamrielMap.jpg
Skyrim is about 1 half the size of Cyrodiil, yet bigger than morrowind. (Morrowind was pretty small geographicially...)
So wouldn't it be in their interest to call TES5: SKYRIM, but have the game in all three provinces to the north, hammerfell, high rock, and Skyrim? That together is barely larger than Cyrodiil?
It's possible...Would it be cool? I'd think so...DLC? Maybe...