Then Sam and Dean drive away in separate cars.
This is more of a final episode ever idea, based on a set of theories yourshipmentisin sent me.
Demon!Dean is permanent.
Sam starts the Men of Letters over, and invites others to join. He acts as a sort of Bobby figure to them and hunters who come to them for guidance. He also marries, and lives a happy life like he always wanted.
Dean still prefers the open road, so continues to hunt every evil son of a bitch out there. Because when you’re restless and immortal but still Dean Winchester…why not save some people and hunt some things.
Castiel finds a solution to his grace and is a full angel again, but instead of returning to heaven, he prefers to stay by Dean’s side.
That’s about as happy of an ending you can get if Dean is not able to be cured. There was a second half of what happens after the credits, that I may add on later.