I personally would absolutely LOVE it if TES VI was set either in Black Marsh or Elsweyr. Mostly because the beast races are my favorites. Since Elsweyr is kinda small on its own, though, I'd combine it with Valenwood.
However, I deem it most likely that the next game will be set in Hammerfell. I know that Skyrim borders on Hammerfell, but for some reason the amount of Redguard stuff in that game indicates that to me. Also storywise Hammerfell would probably work pretty well, given that the Redguards are the ones most openly defying the Thalmor.
But most of all I want to see more cultural diversity again. Skyrim just felt like 70% Nords and 30% Imperials (not taking the Dragonborn DLC into consideration). One of the best things about Oblivion was that you could meet all kinds of people from every race scattered throughout the whole province. Sure, Cyrodiil is located in the center of the continent and its capital is also the capital of the whole continent, while Skyrim is kinda at the edge of nowhere and has hardly any political importance, not to mention the harsh climate, but still...
I admit I don't know much about Hammerfell's geography, but I always thought of it as being 90% desert. And I'd really like to travel through a land mostly covered in forests instead of wastelands for a change. Especially considering modern graphics.
Playing Morrowind for a while now also made me realize another thing - Skyrim and especially Oblivion have a HUGE lack of ways of transportation. Sure, you can quick travel in these games, which kinda makes everything else unecessary, but it feels way more immersive if you pay an NPC to take you with his Silt Strider to another city or if you have to cast a spell. The boats (only 3 places to go, I think) and carriages (you can only take one from 4 places in teh game if I recall correctly) in Skyrim are a joke compared to the different means of transportation in Morrowind:
- Teleporting from one Mages Guild to another
- Silt Strider routes linking most major cities and villages in the south and west
- Boats can take you to every settlement along the coast
- Mark and return spells to take you back to a specific spot
- ALMSIVI intervention to teleport you to the nearest temple
- Divine Intervention to teleport you to the nearest Imperial Cult Shrine
- Teleporting between fortresses if you own the required keystones
I didn't even miss instantaneous quick travel in Morrowind, something that even surprised myself.