You kidding? Bethesda already stated they aren't doing anything as big as Shivering Isles (very sad

and I hope they change their mind), and Bethesda has never done a whole province as a DLC.
Together, these circumstances can tell us that Summerset Isle as an expansion will never ever happen for Skyrim.
At best, we might get Daggerfall, Roscrea, Atmora, or a part of a nearby provicence, or another daedric realm (<_<).
I love Summerset Isles but I think it deserves to be a stand-alone game.