I don't see any good reason why Skyrim should be a sequel on Oblivion! It was rumored before the announcement of Skyrim, so in my opinion it will have nothing to do with Oblivion or go further on it. (this is my opinion i ment no personal assaults with this)
The Oblivion story is pretty finished anyway. What do you want to had? Nirn is access off to the Daedra, Martin has become Akatosh' avatar, it's finished. It can't be a direct sequel, what would you add?