Personally i wish for additional landmass, if not whole provinces (though High Rock wouldn't be too large, nor Summerset Isles if it is scaled down) then at least parts of them. Visiting Morrowind would be nice, Vvardenfell especially. Seeing how it has changed over 200+ years...
Shivering Isles was pretty good but not more Planes of Oblivion please, unless it is just a visit for one quest.
Skyrim has a lot of content already so i don't think it needs more, other than allowing us to divorce, remarry and/or have multiple spouses, own stores and shops and stuff, fund building cities or some cool stuff like that.
Besides Shivering Isles and Mehrunes Dagon's plane, has the series taken place in any other Daedra's plane ? Love to see Azura's plane.