I have a feeling this will get locked, because the official location/setting speculation thread is still open (yes, I realise that one doesn't have a poll).
But anyway: I'm going with Black Marsh. 1. Argonians are popular. 2. Black Marsh hasn't been directly involved in the Great War, so there's a lot of freedom there for how the story is developed, and how the Empire fits in (which has been a major part of all TES games). 3. The conflict between Black Marsh and Morrowind brings the game in touch with the series' history. 4. There is plenty of scope for variety in the landscape, the settlements, and the creatures/enemies.
I think the fact that Summerset Isles and Hammerfell are the "obvious" choices, having such a close connection to Skyrim's story, counts against them. None of the previous games have been direct sequels from the previous installment.