Man and Mer have dominated TES. I'm willing to bet that it will stay that way.
The only way I think Elsewyr or the Black Marsh could be successfully done is if the game showcased two regions.

I think they could spectacularly pull off an Elsweyr game. Especially with with the popularity of Assassins Creed, Jade Empire, and Prince of Persia.

Black Marsh, though, is more iffy.

The games been out a week and people are already talking about the next TES.

Actually, they were talking about the next TES before Skyrim came out.

Elswyer could be done rather well I think, Argonia though...ehhh no.

http://www.gamesas.com/index.php?/topic/1279536-next-tes-location-vote/