Do we need a thread like for every province or something??
Well, it's much too early to guess with any real accuracy. I really doubt Bethesda has even thought about it, especially since Fallout 4 will likely come first.
But, since this is a speculation thread, I would think Summerset Isles or Black Marsh (to do something very different from OB and SKY) or maybe a Valenwood & Elsweyr together. (since both are rather small, and it could introduce a war between the two)