Personally, I think this should be the show's last season. Kripke planned for five seasons in order to tell the story the way he wants, and I want him to end the show on his terms without much of outside influence of adding on, which he clearly voiced his opinion on going out on top several times. So I don't think there should be a sixth season unless, of course, it's an extension to this season's arc. Because really, how can you top the freaking Apocalypse?
But I will say, I wouldn't mind having telemovies or even webisodes afterward. Perhaps something Castiel-centric, or with "the plan" of the angels? *nudgewink*
Hell, a prequel series with the adventures of young!Mary would be kinda awesome, don'tcha think?