I am completely, blissfully unspoiled for tonight's episode (and please, for the love of Chuck, help me stay that way), I don't even know the title, but I have some thoughts about what could happen, based on the foreshadowing we've seen this season. Here are some possibilities:
1. Somehow, Sam will use the Book of the Damned to save Dean. I know, this one's kind of obvious. But not only because the story arc leads directly to it, but because it's been foreshadowed as well. We've already had one episode in which Sam literally saves Dean because he refuses to burn a book, written by a nun who had a penchant for using tissue and blood as media, even while Dean yells at him to burn it. How can that not be foreshadowing?
2. Something awful, possibly death (or "death") will happen to Sam in the process, possibly as a requirement of the spell to save Dean. Sam demonstrated that he is (once again, literally) willing to die in order to save Dean, when he was bleeding into the Werther Box. He's pointed out that he loves his life now (thus it would actually be a sacrifice to lose it, something he hasn't always felt) but he doesn't want to live it without Dean. And again, this is kind of a big "duh," but it's also something Show is explicitly highlighting.
3. Sam will kill Dean. I don't see it. Show has hinted that this is possible, but not as much as they've hinted at other things. And let's face it - Dean was killed at the end of the previous season. I don't see them doing that again so soon.
4. Dean will kill Sam, or at least make a pretty good attempt. In my humble opinion, this is very likely. Dean's been given two opportunities to kill Sam avatars since he was cured of his demonhood. The first time, when Rowena attacked him by using her powers on the long-haired pool-playing college boy, Dean was able to resist the Mark and let him live. The second time, when he shot longish-haired Cy Styne, the studious little brother who didn't want to join the family business, he did it without regrets. Foreshadowing city, my friends. (Also? I kind of desperately want it. Because I'm messed up.)
5. Rowena will die. I think the odds are at least 75% against her surviving the finale.
6. Crowley will die. I'm 50/50 on this one. On the one hand, all signs point to yes. On the other hand... I don't know. I just have a feeling. I suspect that if he does die in this episode, we'll find out later that he was only mostly dead.
7. Castiel will die. Nope. I don't see it. Like Crowley, he could "die," but I don't think he'll die.
What do I want to happen? I want it to be awful. I want 42 minutes of guilt and terror and blood and horror and grief. I want Dean to see how far Sam will go for him, what he'll sacrifice. I want Dean to do something terrible to Sam, to make them "even" for the terrible thing he thinks Sam did to him. I want Sam to be the big damn hero. I want somebody to sob and hold somebody else's limp bloody body.
I want it to be 8:00 already. (sigh)