Since they're working on Faloout 4 I doubt we will see an Elder Scrolls VI as well, but I would be extremely happy if I was proven wrong.
My guess is pretty much the same as yours - most likely it will take place in Summerset Isles, or maybe even Valenwood or Elsweyr. Only thing I'm sure about is that the next game to revovle around the Thalmor.
Hey guys, correct me if I'm wrong but I'm pretty sure that Bethesda leaked that it would be set in Argonia (Black marsh). So with that in mind there I think it will have great deal to do with the hist and a feud between two houses with different opinions on their culture *cough* Stormcloaks *cough*. But that's just my guess.
I have a feeling that were going to be going from tundra to desert, and this makes me think that Hammerfell will be the setting. We didn't really get a good look at Hammerfell in Elder Scrolls 2, not in the way that we've seen Vvardenfell, Cyrodil or Skyrim anyways. The geographic difference would be enough to make the game feel new, and fresh, but it's still a province populated primarily by a human race, so it would be marketable to the masses (lots of people might be turned off by having the game filled with lizard-people, for example).
Story-wise, the Redguards are separate from both the Empire and the Dominion, and in close proximity to both. It would make a good setting for finishing the story that Skyrim started, with the impending 2nd Great War. Summerset, too, would make sense, but lore-wise isn't it populated almost entirely by high elves? It wouldn't make much sense to have a bunch of different races there, at least not at this point in time, what with the Thalmor's views on eugenics.
Elsweyr and Valenwood are both good choices too, but I feel that Hammerfell is more doable right now, with the limitations of technology anyways (no migratory trees). Given all this, I feel that Hammerfell is the most likely setting, and that the story will have something to do with the Thalmor. I just hope that they go back to a Morrowind style of story telling; you aren't a near demi-god like the dragonborn, and you can actually justify taking your time while doing the main story. Skyrim just felt so rushed, half the time.
I thought in Morrowind you WERE a god.
But other than that I agree with the rest of your post. Maybe you could help fight off the Empire and the Dominion from Hammerfell, or perhaps even join them.
Summerset Isles could work. We don't know how far into the future the next ES will go, be it 200 years, or 1000 years. A lot can change in that time. Everyone might have even forgot about the Great War, since no one even remembers the Oblivion Crisis!