Honestly, It's going to be either the Khajiits homeland, or the Argonian homeland. (Elsweyr or Black Marsh)
Making statements like these is the best way to set yourself up for future disappointment. The next game in the series could take place in the Summerset Isles or Valenwood just as well, or almost any other location Bethesda would like to dream up, really. And setting a game in the Summerset Isles probably wouldn't take nearly as much effort as setting the game in an utterly alien location like Black Marsh, which might not even appeal to people who aren't real fans of the series (ie anyone who frequents this forum).
As an argonie fan or more broadly a beast fan I'll keeping my fingers crossed for something exciting like Elsweyr but at the same time I think I'll brace myself for the moment the other shoe drops...
EDIT: It didn't occur to me until I actually wrote this post (it's late where I live, gimme a break) but http://www.gamesas.com/topic/1361134-official-beyond-skyrim-tes-vi-8/.