Why would areas not used in any of the TES series not be "traditional"? High Rock and Hammerfell were used in Daggerfall (which, IMO, is the best of the whole series). Why not Blackmarsh, Elsweyr or Summerset Isle? The whole area of Tamriel is "traditional". Sales has nothing to do with it. They didn't re-do Hammerfell and High Rock in Morrowind for sales reasons, they made Morrowind to be in a province not yet used, and the same for Cyrodiil in Oblivion. Using your logic, Skyrim is not traditional because it hadn't been used until now.. Note, that the races of Tamriel come from different provinces.
You misunderstood me. By traditional fantasy I mean more typical fantasy looking locales and enemies. The more "different" the look of the game the more sales you may lose to people who only like traditional fantasy. Go to any hardcoe RPG board and you will see a ton of people who hate anything that strays too far from traditional fantasy. Bethesda employees have made comments about this type of thing in the past.
Cyrodiil and Skyrim both offer traditional fantasy locales and enemies. A game set in Black Marsh, Elsweyr or Alinor would be much different and less "mainstream," for lack of a better word. I'm not saying it's impossible that they set a game there, I just expect them not to.