I think what would be great is for Dean to now slowly give into (or be forced to give into) the Mark of Cain, to the point that he becomes something new, not a demon, but not human, angel, nephilim, anything, and the season finale involves him about to go on a major madness spree, ala Castiel with all the souls or something similar.
Bright light flashes, someone grabs his arm, and the mark just evaporates and he gets knocked out. Sam looks up and we see a person surrounded by the archangels we know to be dead or trapped. Metatron is free with them as well, and he speaks to Sam and thanks him and his brother for completing the necessary steps to bring God out of hiding, but that unfortunately it may come with a cost.
Final season is a severely weakened Dean, Sam, and Castiel rounding up past friends and even some foes, to stop the real God Driven apocalypse.