Yeah but with the very first episode having Sam being tainted by YED, and its growth throughout the seasons, wouldn't an ending like that be a sort of paint by numbers. Tough in truth pretty much any ending they do, will be something that doesn't surprise.
As to Sam being the host for Lucifer the reason I say no, is purely a production one.
Look at all the previous big bads they have had, and look how little they have actually been on the screen.
If Sam is possessed by Lucifer, first the story is no longer about Sam and Dean, its about Dean and Lucifer. One thing Kripke has always been clear about is this is a show about the brothers. So just on that I don't think they would go there for an all out possessed character.
Now is it possible they will build to a point where Sam would willingly give himself over to Lucifer? I just don't see it.
Sure we have seen Sam be addicted to power, but we have also seen his utter hate with the forces of hell (remember he only put up with Ruby because she was pretending to be against the forces of hell). It was his hatred for what was done to him, for what happened to Dean and what could still happen to Dean that has pushed him over.
On the production side they have never done a long term separation between the guys, in fact how many episodes have they not had some form of direct contact? I don't think there has been a single one. I don't know if the producers are both brave enough and good enough to keep two fully separate stories going. I just haven't seen it.