I'll let you guys in on a little secret: Pretty much every.single.video game in the past 15 years has had background landmasses, trees, mountains, etc. This doesn't mean anything, and it's a little insulting to the forum's intelligence that people here actually think there could be DLC from it.
No, it's not. Those land masses look ready to be populated. Maybe they are there as a courtesy towards mod developers. Or maybe they did them while doing Skyrim itself to save time later.
My money's on Cyrodiil, however. The White Tower being there is more than suspicious. Also, Skyrim is a
lot stronger on Oblivion references than that game was on Morrowind ones. I still think it will be about kicking out the Thalmor and the Imperial City is a. the right place to do just that and b. a place they already made so it's probably rather easy to import/update it.