Honestly I thinks it time for supernatural to be done. They have just done about everything they can do. I think there should be two more 16-episode seasons to wrap up the show. I think for season 14 the big bad should acutally win, the the apocaplyse takes place. For the 15th season, I think it should be about Sam and Dean wandering a post-apoctalyptic world they failed to save, Castiel, Mary, Jack truly dead along with everyone else they know and love dead leaving them with just each other just how the show started.
They would also visit memorable locations from earlier in the show (their house, a town they saved, Dean's Boys home), all these locations in a wrecked form. I think it could be about them accepting that the world would end at some point and they did the best they could.
At the last episode, I think they should both die, Dean holding Sam in his arms and telling him that they did good. The last scene would be of Sam and Dean in a shared heaven, driving the impala down a road.
Anyways, that my two cents of how the show should end.