I'm not very fond of the idea of the next TES being set in the entire continent. It's better if their focus is centered in one province at a time, so it can be a lot more detailed in every aspect: lore, geography and story.
As to where they should set it, I hope they do it in Hammerfell, it's the only kind of setting we haven't had yet (we had swamps in other games). Plus, it would make sense they'd finish the northern provinces before moving on to the southern ones. (High Rock would still be missing; I'd like a game there too)
I'm still of the opinion that we're likely to see TES:VI set within the Aldmeri Dominion, probably during a time of upheaval as people rebel against the Thalmor. The Summerset Isle would be a very different environment than Skyrim, and the high magick presence allows you to do a lot of weird stuff with unnatural animals and situations. The only question is how you justify having so many non-Altmer there, when the Thalmor's whole deal is racial purity.
I could be wrong, but wasn't there some cleansing where most other races were killed off in the Summerset Isles? That seems like it would get rid of the race diversity and be boring, so that seems unlikely. Unless they fast-forwarded until the Thalmor are already defeated lol.
Hammerfell and Highrock I think would be cool, but I think they might shy away from those because of Daggerfall being set there.
So that leaves. . .Argonia/Black Marsh, Elsweyr, or Valenwood. Valenwood has the whole moving tree-cities thing too, maybe the devs wanted be be ambitious with the new gen consoles and see if they could run that :D