I don't think it will be in the game, nor in any expansion. We've already explored Solstheim. And its environment is very similar to Skyrim's.
In an expansion, I think we'll see something more unique. Something that differs a lot from Skyrim's environment.
Just look at how big the difference of expansions were between SI and OB, and Morrowind and Tribunal and Solstheim.
I presume the same big difference for Skyrim's future expansion