Well, considering this beautifully useful map:
The only thing this map is missing is the Dragonborn label on Solestheim as well as Bloodmoon. I think it is apparent that we need to visit somewhere in the South, as that region has been sparsely covered thus far. Given the story about Alinor and the Thalmor I think the next game should visit Summerset Isles, Valenwood, Elsweyr, or, if Bethesda can pull it off, all three of them. two would make up an area comparable to Skyrim's size, so three would definitely be a feat, bigger than Oblivion even.
Another idea I have is visiting the whole of Hammerfell and High Rock, the story would revolve around the return of the Dwemer.
A final one is visiting Black Marsh and Southern Morrowind (not Vvardenfell, so it would be new ground), the story would involve the warring between the Dunmer and Argonions and another Akaviri invasion and the fate of the Neravine after his expedition.