Morrowind (or at least part of Morrowind) seems the most likely addition if landmasses are going to be added, because it already has an in-game road connecting it to Skyrim. I personally don't think they're planning to add all of Morrowind, because the size of places like Vvardenfell looks highly abstracted, but the regions along the Skyrim border seem a likely candidate.
Just because the landmasses exist doesn't mean they'll be used. The in-game map is actually based on the real terrain, so it's possible they were just added to make the map work, or as a gift to the modding community.
I'd like to see the Thalmor metaplot advanced, with more clues about what might happen to Tamriel in the next few centuries.
Alternately, a purely exploration/adventure based DLC set in Atmora. It would require very little work relative to making somewhere like Blackmarsh (which sounds like a tropical swamp on crack, virtually everything from the trees up would have to be new).
Just because the landmasses exist doesn't mean they'll be used. The in-game map is actually based on the real terrain, so it's possible they were just added to make the map work, or as a gift to the modding community.
I'd like to see the Thalmor metaplot advanced, with more clues about what might happen to Tamriel in the next few centuries.
Alternately, a purely exploration/adventure based DLC set in Atmora. It would require very little work relative to making somewhere like Blackmarsh (which sounds like a tropical swamp on crack, virtually everything from the trees up would have to be new).