Given how bored Jared has acted most of the season, and his stated desire for Sam to go evil, this is not a bad scenario at all. I agree they have to do a miniseries type of final season - one arc (which supposedly they've claimed they will do) that takes them through the series finale.

Sure, they could have season five end with Samifer - though that wouldn't be ending the Apocalypse scenario and they swore they'd do that; or, something else having taken over Sam as a remnant of the dust-up with Lucifer. Then Castiel is either completely fallen, or is given the choice to stick with Dean to hunt down and fight Evol!Sam, with the series ending at the point where Dean saves Sam.

Yeah, works for me. ;)