Mainly posting this so people have something to point at and laugh about how wrong I was.
No spoilers, just wild guesses. That maybe sometimes conflict with each other, but what the hell.
-Epic Pain. Emotional and otherwise. Because it's a finale.
-Castiel's gonna Fall.
-Dean (or Sam, but more likely Dean) will get killed and Castiel will screw with time hugely to bring him back, then Fall (completing the Adoption proceedings of sacrificing his soul for the good of the party Winchesters.)
-Or maybe not, but SOMEONE IS GOING TO SCREW WITH TIME. Hugely. You don't leave a big "By the way, the paradox rules have been turned off" sign up and not do something more with it than temporarily raise the Titanic. BRING ON THE CAUSAL LOOPS!
-SOMEONE'S GONNA DIE, or experience a death-like event. I hope it's not Bobby, because at this point, of all the members of the Team, Bobby is "Most Likely to Stay Dead". Sadly.
-THE SOULS. Dean's doing a crap job at investigating the souls, and Death could totally show up for a Situation Report.
-RAPHAEL will either reinstate the Apocalypse, or get ousted and replaced. Possibly by Crowley. O.O Who might then reinstate the Apocalypse for giggles and MAXIMUM CASTEL ANGST at having helped him.
-SAM'S WALL is going to crack. Hard. It will be a VERY BAD THING
-MONSTER EGGS. I don't know, but it's not gonna be breakfast. Looks way too much like ...I wish I could remember the name of the big mother Lovecraft monster. The one that looks like bubbles? Shoggoth? No, that's one of the more minion-level Lovecraft monsters. AH!
Shub-Niggaruth. Yeah. If Lovecraft's estate wasn't so intense about copyright, Eve could totally be Shubby. The one with all the kids. It's like, even though the host body is dead, the Mother is still on some level alive, connected to her kids, laying eggs, bringing more potential monsters into this world. Almost like an interdimensional portal or something... Wild. Bears a LOT of pondering. Wish I had more time for it.
-Speaking of which, TRANSDIMENSIONAL GATES. There's got to be more than one alt!verse, and having a handy spell to jump dimensions really can't be left laying around long before some plot trips over it.
-BEN AND LISA. Big old unprotected targets out roaming the lower 48 somewhere. What are the chances they're going to make it through the finale whole and happy? Not that great, provided any of the Winchesters' adversaries have any tactical sense. Want to distract and distress the Hero? Go after the Love Interest and the Dependent (if the Baby Brother is now too much of a badass to handle). For that matter, Dr. Visyak and her 'history' with Bobby might make her a target as well, though I'm less worried about her as she is my Mary-Sue a badass academic who can take care of herself as long as Dean doesn't break any more of her toys.
-FAIRIES came and went pretty darn fast for them not to be coming back nasty. Two episodes left, might not be until next season, but with Mama Monster gone, there's an awfully huge vacuum in the power balance of the universe the Fae might try to exploit.
SUMMARY: I HAVE NO IDEA WHAT MIGHT BE COMING, BUT THERE COULD BE A LOT OF IT. O.O
And that's enough madness for the evening. Feel free to share your own theories in comments but NO SPOILERS AND NO REFERENCES TO ANY PROMO MATERIAL PLEASE!!