How will the next game explain who won the civil war, or whether the Emperor was assasinated? The obvious solution would be to jump so far into the future that nobody seems to remember or care, I suppose. Mind you, what did they do for the Mass Effect games if someone played the second or third game first?
Anyway, on to the suggestions.
Hammerfell. Not too mysterious a region so that the game would spoil it, but interesting enough (with its own take on the divines, and other ways of life).
So long as it wasn't empty, I dont mind the deserts. Plenty of oppurtunity for ruins and the sort. Also, this land has a history with the Dominion, so maybe we could do a continuation of the Thalmor and their conquest to dominate.
Regarding Valenwood and Elsweyr, arent they both quite small? I suppose you could have both of them, but then woulden't a world full of Khajiit and Bosmer be really strange? Especially if you didn't belong to one of these races?
Now the story. Though I have never properly played the game, Oblivion sounds as though it had a better MQ than Skyrim, especially as there was a political element involved. Large battles would be great, and I'm sure PC/Nest gen consoles could handle them. The other problem with Skyrim: Sneak characters didnt feel fitting for fighting Dragons. Lets have a main threat that can be contered equally by any playstyle. Again, politic-based MQ would work here. Also, a chance to choose an evil side in the main story, maybe even become the Antagonist, would be epic!