What else would it be? Solstheim is the perfect place. I know it was in Bloodmoon, but still. Snow Elves? Riekling followers? Yeah, it's Solstheim.
Solstheim would be cool, but I hope they do somewhere else. Seems to me that Solstheim would have the exact same feel as Skyrim does. I want somewhere with a different atmosphere (look at Morrowind VS Bloodmoon, Oblivion VS Shivering Isles.)
You know, some variety! Solstheim seems to be the most boring option to me for possible new areas.