My estimation
>Skyrim
0,01%
Very unlikely, even though a part of it might be accessible later through an add on
>Cyrodiil
1%
Very unlikely, even though a part of it might be accessible later through an add on
>High Rock
15%
Not very likely, would be my personal favorite though. Lots of mountains, forests, castles and court intrigues.
>Summerset Isle
35%
There is a good chance considering that Skyrim ended open towards the Thalmor and the upcoming war, we also don't know what the Thalmor want for sure
>Valenwood
20%
Possible, but my least favorite since, well, it's a fucking forest. A combination with the Summerset Isles would be possible to cover the entire Aldmeri Dominion
>Morrowind
10%
Would not rule it out yet, we might see the parts of it that aren't Vvardenfell, and even if the title isn't Morrowind again we might be able to go there as it is close to Black Marsh, if not in the vanilla version for sure in the expansion
>Elsweyr
15%
I don't know, it's just desert, with nothing really there and everybody would be a cat
>Hammerfell
70%
My favorite. Reasons: The quest with the Redguard hinted about some shit going down in Hammerfell, it's at the frontline of the war, it's a diverse country with different landscapes and noble houses. The only reason why they wouldn't do it is because it was already covered in the Redguard adventure game but on the other hand they could just build on that
>Black Marsh
40%
Media seems to think this will be it and there are a lot of reasons why it would be likely. An argument against it would be that it's just swamp and the inhabitants are Lizards who are not really as fascinating as humans or elves and while it's nice to have the animal races arround, to have a whole game based arround them might be a bit too much