I can picture myself opening the box on release day and seeing "download Solstheim for only $4.99 today".
As to why they really wouldn't, it's a lot of extra work(Bloodmoon had, what, 40 hours of content?) for something that isn't part of Skyrim. I can esily see it being part of the game, but would it be more of an after-thought or are you expecting an island with nothing to rival Bloodmoon in content?
Well it can't exactly be a part of Morrowind, what with it currently having no government, and it being volcanic wasteland and all.
There is no more Morrowind. The southern half of Morrowind, a.k.a. the part not covered in volcanic ash, has been taken over by the Argonians, and the northern half is, well, destroyed. The Dunmer fled to Solstheim, and considering it's so close to Skyrim, I don't know why Bethesda wouldn't include it in the game. I'm not talking about adding it in as an afterthought, I'm talking about just having in the game, no more of an afterthought as anything else. It would be really interesting to see what the Dunmer have been able to do to Solstheim, and it would be a nice change of pace from Skyrim.
Of course, I also want Bethesda to include the island of Roscrea, considering it became a part of Skyrim around the time of Oblivion, but I doubt they will, considering it's so far north. But Solstheim is nice and close.