I think if they intend to bring Cain back, they might've signposted it too obviously by not showing us a body.

That's a good point, I actually didn't even really think of that! What do you think it could be concealing? I read above that someone's idea was that Dean had trouble getting out of the Devil's Trap, I think that sounds like a really neat (and possible!) theory.

I'm really hoping Dean turns into a demon again because I'm a Horrible Person but I feel like they've kind of been foreshadowing it the same way they did in S9, filming it so Dean's eyes are dark/black/in shadow, etc.