If Molag Bal is indeed the antagonist then we may see his plane Coldharbour:
"Coldharbour, the immense realm of Molag Bal, is also one of the most inhospitable realms, consisting only of charnel houses and vast slave pens. Any mortal found here is captured and placed in one or the other. It resembles Nirn except colder, darker and more beaten and violent; according to some a premonition of the future of Nirn itself."
If this is true then we effectively get to explore the whole of Skyrim again but this time a whole lot darker and twisted.

