Nobody make fun of me! *warns*
Okay, so I totally wouldn't mind a scenario where everyone thinks Castiel is dead and for all intents and purposes, he is. Only, as he was consumed by the Leviathan and losing control, he knew he had to take desperate measures to survive so that he could keep is promise to Dean: he would make things right.
So he transfers a small part of him, a bit of his very core, and buries it in Dean. Dean is unaware of all of this.
Anyway, the months go and little bits of Cas are starting to leak through without Dean being aware. Sam picks up on it, but isn't sure if he's hallucinating or if it's really happening.
Eventually, Cas will be strong enough to take control. The last thing he wants to do is override Dean, but he's not strong enough to find a way to recreate Jimmy as his vessel, and he's too strong to stay hidden. So in a temporary move as he figures out what to do, Cas takes over Dean, and Sam is left, with his broken mind, to try to figure out what is going on.
After some angst and fun, get Misha back to play Castiel and the three of them can navigate their next move against the Leviathans. (Because I can't go too long without my Dean.) Do they trust Cas again after what was done? Did Dean let Cas take over? Did Cas not get permission? Any lasting effects on Dean? What about Sam? Can they ever be allies, and even more, friends again after what Cas did to Sam, Dean, and the world?
Of course, I am a fan and I have my own little biases, but I think it would make for an interesting arc for everyone involved. Maybe I will still write it someday. But I wouldn't be against the show exploring it ;)