The Season 8 promo clip also hinted at the storyline for this season. The prophet Kevin from last season found another word of God tablet that talked about closing the gates of hell which would banish all demons from earth. Thats pretty much an exclamation mark goal to cap the Winchesters legacy. I do not expect this to be a season arc, but at least a two season arc like the first two seasons of Supernatural with the demon Azazel. It could also be dragged out to the tenth season as an overall large story arc. I cant see them getting rid of demons outright from the series, and I expect that to be a series conclusion type of deal. As for other story lines, Sam's love life and Dean's purgatory experience will be at the forefront for at least the beginning of the season. Castiel's return and dealing with Bobby's absences will most likely also be explored.