Black Marsh is a big possibility since it hasn't been the main location in a game ,unless you include Stormhold but most people forget about TES travels games, and it also has some of the perfect Lore setting for the Dark Brotherhood and the Shadowscales. It also would have the possibility of the Argonians trying to invade Morrowind for a bit of revenge. The biggest thing that would be a pain to deal with or for the developers to incorporate is that the Argonians would have worse racism than the Nords and would be bigots towards the character if he/she wasn't argonian and that isn't counting how hard it would be to put a population of non-argonian NPC's and make them fit into the story and Lore and if your character is a non-argonian how they ended up in Black Marsh. just a bit of food for thought. (P.S. it would be a really weird game if almost everybody you meet is a walking talking lizard.)
I think there are two big candidates, with a few caveats.
My dream will always be for a TES game set on Yokuda. However, seeing as we don't know how much (if any) of the nation remains intact/inhabited, and as it's not a province on the core continent of Tamriel ... I'm not particularly hopeful that it'll happen.
I would probably go with Valenwood as my next most preferred location. I don't know a whole lot fbaout Bosmer lore, but I'd love to get a first-hand look at it. Between the Imga, the Wild Hunt, the Precursor, the Green Pact, and a load of other content, there is definitely no shortage of worthy material to feature. I'd love to see what BGS could do with a jungle/forest setting. They have the technical capability to do it justice now, unlike Cyrodiil.
I'd be totally okay with literally any province though. They all have a lot of awesome lore to delve into. Khajiit Temples in Elsweyr, Argonian tribes in the Black Marsh, the Thalmor/Psijics/Crystal Tower in the Summerset Isles.... I'd also have no qualms about revisiting the region(s) covered in Daggerfall. Will never say no to anything Redguard-related, and I think High Rock and Orsinium would also be worth a revisit.
Well, the story does not have to be linear... Like Imperial/Stormcloak dilemma
In fact, I think Valenwood/Elseweyr is quite a likely location. After watching ESO walkthroughs, the majority of the Dominion questline and its most important (IMO) parts took place in that region. These provinces also provide the bulk of the Dominion forces, so crippling Thalmor there will cripple the Dominion as a whole.
I'd like TES VI to be located in Elseweyr. The story would be like this: The Khajiit realized that the Thalmor deceived them when they said they used "dawn magic" to "restore" the moons. The player would be someone send by the Empire to help the Mane to fight against the Thalmor and depending of his/her actions, Elseweyr can become an Imperial province again or an independent one.
Another cool place would be Akavir. It would be great to play as one of the races that live there.