I hear Elsweyr, Valenwood, and the Summerset Isles being bandied around a lot. I think any of these are viable, although not so much the Summerset Isles with the Thalmor as they are now. Now, what might be interesting, and possible, for the next game... would be if they did all three of them at once. Three states? Three distinct environments? Lots of different cultures? HUGE World? Possible. Maybe they'll only do two. Probably, they won't do that. But it would allow a new direction, and I think it'd be pretty interesting. It doesn't have to be those necessarily, but a lot of interest has been expressed, so hey, why not?