Summerset -freakin- Isle, please
Many people vote for Black Marsh in the "next-elder-scrolls-game-contest". And while that would be really cool, I think it would be too dark and too... swampy (:P) for a game. I mean Skyrim is a dark game. A Black Marsh-game would be that too. I just think it's time to go to a really bright, magical and shining land!
Summerset Isle could probably be more diverse, with mountains, meadows, forests, magically abnormal landscape and possibly a (black) marsh area somewhere too! Here are some images of cool landscape variants that could be used in some extent:
http://www.epilogue.net/cgi/database/art/view.pl?id=109578
http://aliceleegraphics.com/_img/water_island.jpg
http://suptg.thisisnotatrueending.com/archive/14749353/images/1304053210610.jpg
The cities could made of either elven, coral (sload-ish), crystal and even insect architecture. Very interesting. Very alien (like Morrowind), but still being a lot more bright, and thus delivering a different theme and feeling. It's still unique!
The Crystal Tower would make a perfect "Mages Guild"-faction.
There are tons of possibilities for imagination, as it is the magic land of the altmer. (Wouldn't it be cool to encounter a huge crystalized crater with floating rocks and a floating island above, where some magic experiment has gone out of hand. Or perhaps the land is just so heavily affected by magic in places that it behaves really awesome. Waterfalls going backwards? ^^) Since daedra worship is becoming increasingly popular there, I can imagine high elven mages walking around the streets of Alinor with their daedra servants. There could be tall wizard towers spread across the land, some having a small village belonging to them, others being remotely located.
And the main quest could be a sload invasion? ^^ Or something a bit more imaginative. Bethesda could probably think of something better than me when it comes to that!