Personally, I would be pretty pleased if it gets to the point where a "Godless universe" can't be maintained or some shit, and since angels don't actually have souls none of them are up to the task, so Dean has to become God. And Sam can live the rest of his life like the normal person Dean has always wanted him to be. It would be hilarious because Dean is like the last person you'd expect to be God, but that makes him a great choice.
And he brings back Crowley as his Prime Minister or whatever. I realize he left the show under a bit of a cloud, but I think he deserves his spot in the finale, because he really propped up a lot of episodes in some of the drier seasons.