Personally, I think the original main story of Skyrim should be switched with Solstheim. It is far better, far more intriguing and interesting. [mind you I haven't beaten it, quite just yet, except the main storyline with another character, but haven't beaten the Solstheim questline just quite yet].
But hands down Solstheim is far better storyline, a far better world. Solstheim is my Shivering Isles. But even then Oblivion was more fantasy than Skyrim. All my fantasy elements in Skyrim were all hiding away in Solstheim this whole entire time.