But expectation is that this will end the arc. So I would assume that Sam and Dean will survive, but their roles as the vessels for the last battle will be resolved, and then there will be another season (presumably sans Castiel, which sucks, as he's a fun character).

I have no idea what story/big bad they could possibly come up with that wouldn't be a letdown. It'd have to be some other pantheon or something, but the only power anywhere close to angels and demons they'd faced was Loki (who turned out to be an archangel). There've been a couple other things of note (this season's ancient witch), but it's always been about the demons.

You could still have some demons interloping, planning for the next try at the apocalypse, but it can't be the focus of the show next season. (You could easily bring back Crowley, and I wouldn't put it past Ole Yellow Eyes to have had some personal plan beyond getting Lucifer out, something to do with all the kids beyond just finding and preparing Lucifer's vessel.) (You could also have Castiel check back in, perhaps commenting upon the turmoil in heaven and the lack of purpose, etc, but he shouldn't be enlisting the Winchesters. This season needs to end the demons as working to bring the apocalypse arc. Any future demons must be working their own interests.) They'd need to have something else, some mention in their dad's journal that was some unsolved mystery. Delve into other mythologies, or bring on more magic and stuff. Get into Atlantis or King Arthur or a zillion other things (except aliens). Have some power/creatures that has been held down/kept in check during the angel vs demon days, but is not a threat again. (The reapers used to seem to be solo, but I guess are all just parts of the Horseman Death.) Hell, have someone from their mom's family show up, having been focused for years on some distant threat and having heard through the voodoo line about his/her relatives. Bring on something major and the show can live on, but drag the main myth arc on and you lose all the goodwill you've built across 5 seasons. Also, if he survives, add Bobby to the main cast officially.


Also, best line of the ep: (paraphrasing)
Young John, yelling at wife and (unknown to him) his sons: So help me I will turn this car around!
Dean: Awkward family trip...