OK, I'm just brain-farting here, but as you were thinking about the last episode, I think that would be a really interesting take on things. Rather than going for the full serial killer deal though, maybe examine the body horror behind someone inherently good getting some kind of thrill / sense of empowerment from being possessed? Maybe John while inhabited by Michael? Sam with Meg (all sorts of yummy Wincesty possibilities there - but what did you expect from me, srsly! lol)? Bobby?

Also, I would love to read your take on one of the little girls Lilith takes over. Could be very dark.

I'm probably showing my ignorance here as I'm sure there's a wealth of stuff on the subject, but I've never read anything about Crowley's back story. Freshly harvested, Seventeenth Century tailor!Fergus and his enlarged peen might be fun to write huh? :D

OK - that's all I've got. Is that all just too vague? It's bed time, so excuse the lack of inspiration ;)