Okay, This theory is based on personal opinion, previous main game locations and ESO locations.

As first, The locations what can be effaced on instant are: Skyrim and Cyrodiil. Those 2 provinces made up the location of the past 3 games, Thus the chance Bethesda will consider doing it? Almost zero. Then we have TES 3: Morrowind. That game was set in vvardenfall, Not fully morrowind. So that one won't be eliminated on instant. The elder scrolls 2: Daggerfall bethesda used High rock and Hammerfell, It is right now a significant time ago, So the change they do it there? The chance is small, Like morrowind. 

Next there are the summerset isles, The possiblity of bethesda making it there take place will be a medium chance. Remember in TES 4 the shivering isles was a significant addition to the game, Also in the elder scrolls online one place is set there, Auridon. Is there a possiblity Bethesda make it take place on the main land, Summerset isle? Most plausible of all summerset locations.

Next there is valenwood, The possibility of bethasda making it there? Medium too. Not one game, Not even most spin-offs. the only spin-off is ESO, And not a fraction, No. Directly ALL of valenwood. If Bethesda is going to make it the location of TES 6, It will a huge restrainment for the freedom and creativity of the developers since there's already a "Map". Relocations will be too hard i assume, but there it is, In the main games it never featured. It is very hard to say about this one.

Next there is Elsweyr, The chance of this province is actually very big, Like 35%. The main problem with Elsweyr is simply said, Endless desert and jungle filled with cat-like creatures. For all Khajiit fans it will be the ideal location, But the it's a nasty location, The Khajiiti culture is very rich and confusing, Making it hard for the people to follow. Even harder, For the developers, How to make an endless desert an attractive location? It seems very hard and frustrating to develop that properly, But then there's a nice upside, In ESO none of the locations is Elsweyr based, Only Khenarthi's roots. Maybe Bethesda ordered Zenimax to leave Elseweyr alone during the development of the game? Quite an interessing theory. Also there are almost infinite khajiit fans begging practically to make it take place there. If bethesda accepts the challange to make it place in Elsweyr it would an awesome chance for a balance between all needs of all fans. 

Finally we have the Black marsh, Most And this location is almost the same as Elsweyr, The settings are different, But the principe and main idea of the problems are the same, Endless marshes with lizards. How will that be a nice game? Hard. Bethesda really needs the skill to make it interessing to explore, But when they succeed with it, It can be one of the most promising games on the market, However it marshes seem harder to explore than forests, deserts, jungles, mountains and whatever people can think of. Also the location "Shadowfen" was used in ESO. The chance they make it in the black marsh? 25% will be a nice guess. 

To make it simple i'll make a tiny list: 

  1. High rock: 2,5%
  2. Hammerfell: 2,5%
  3. Skyrim: 0%
  4. Cyrodiil: 0%
  5. Morrowind: 5%
  6. Summerset isles: 15%
  7. Valenwood: 15%
  8. Elsweyr: 35%
  9. Black marsh: 25%

Based on this, Elsweyr would be the "big winner" but there are also many other options available, Elsweyr alone would be pretty difficult, So how about Elsweyr combined with Valenwood during the five year war between eachother? Or how about the entire Aldmeri Dominion combined? Maybe Black marsh with Morrowind during the excessive slave trade? Maybe even Elsweyr with Black marsh? The combinations are endless. But that part, There will too much possibilities to speculate that correctly. 

My personal opinion about the location, I personally as Khajiit fan would love to dive into the dept of the khajiit culture, Explore the cities and walk though the jungle, But the map of Elseweyr alone will be too small. Personally i want to see Elseweyr with valenwood combined during the great war. maybe even add summerset isles to it, Would make it perfect and lovely to explore.

What is your opinion? Where should it take place? Do you have a theory? Do not hesitate and place it in the comments :)