Dean as Lucifer would be a shocking twist indeed. But I think it's unlikely that Castiel or Uriel, or even Lilith wouldn't know about it. I mean, Castiel was ordered to save Dean after all.

I don't think it'll be Sam either. Because they specifically said that Lucifer will walk free when all the seals have been broken, and Sam hasn't exactly been imprisoned all this while.

Oh there're still a lot of unanswered questions - the YED's agenda, Ruby's agenda etc.