Three things are very certain based on the spoilers: the finale will be a cliffhanger, at least two unexpected deaths are going to occur, and that there are particular unexpected twists and curveballs going to be thrown at us that even the cast themselves were shocked by.
Now, the unexpected deaths thing really kind of confuses me because with this show anyone can die because they love killing characters. Some are thinking that Castiel might be on that list but honestly, Castiel has died twice in both finales he's been featured in, so that's not really unexpected or shocking. Plus I don't think they would want to enrage the fans further because Castiel is not only a series regular but also a beloved fan favorite, to get rid of him at this point would be ridiculously stupid and they know it. I think Bobby is the only character I can think of that would qualify as an unexpected death. Of course I read that Lisa and Ben are returning in the finale (booooo), and the likelihood of them getting offed seems to be very high. Since you know, nobody would be expecting it. So really, who knows what this means or if they're just hyping things up for the anticipation.
The biggest concern is about Castiel and his fate/involvement in the finale. In fact there are a few spoilers floating about which, while vague, give the impression that something horrible is going to happen regarding his character. And I have seen people speculating his fate and I definitely want to place my two cents into the pile.
First of all, the assumption that Castiel is going to turn evil is, in my opinion, downright ridiculous. There are no indications whatsoever that Castiel has remotely turned Dark Side for whatever reason. In fact, despite how this season has gone, it's pretty clear that he's been struggling with revealing his emotions with what he's been doing during this war he's been fighting. In fact, from the way he's been appearing lately he seems to be more tired and exhausted than ever before with everything, and that's definitely not a sign that he'll eventually turn evil. The exact opposite, really. Secondly, there's the fear that Castiel might turn against the Winchesters. He has mentioned that certain regrettable things have been required of him during this war, which indicates he's had to make difficult choices and decisions that he obviously doesn't agree with but he has to make because of what's at stake, and given the spoilers that he's going to be chatting with the Mother in an upcoming episode, this very fear of what this could mean between him and the Winchesters is very understandable.
Castiel is doing whatever he can to win this war, and desperate times might call for desperate measures if he's propositioned something that might help aid him, but those rarely (if ever) end well (ex. Crowley helping the boys out last season, good at the time but not in the long run) so if this is true, it's definitely going to bite Castiel in the ass.
Now, I'm all for having Castiel's storyline progress further into more morally challenging territories, him questioning himself and his current position and whether what he's doing is right, and so far this season there seems to be much indication that he might be in way over his head with this particular war. However, my main concern is his relationship with the Winchesters, particularly with Dean, because it's been hinted that by the end of this season the main four characters aren't going to be on the same level as they once were in the beginning of the season. Though this is confusing because in the beginning of this season neither of them were on the same page, not even between Dean and Castiel -- because let's face it, Dean has been acting like an asshole towards Castiel for absolutely no reason which makes me sad. So if they split them up further I'm going to be majorly pissed off. They've been not on good terms lately and I hope whatever shifts are gonna happen are for the better and not the worst, but this show loves its angst so you never know what they'll do.
Case in point: Castiel would never betray the Winchesters, not after everything they've gone through together, and he's definitely not intentionally turning evil or against them for whatever reason. Even with some statements like the "power hungry" quote Jared Padalecki made in an interview, because Castiel wants order and peace in Heaven, but it's like, have you even met his character? He's not the "power-hungry" type at all. Just, yeah.
I'm highly concerned over him, is all. It's just nail-biting anxiety over his character because if there's one character I love and adore and care about on this show anymore is Castiel, and if they ruin his character idk what I'd do.
Never mind that this finale will be a cliffhanger, therefore adding more anxiety and worrisome thoughts into ours head.
Those are my thoughts as of right now. Feel free to add your own input or concerns or theories into the mix.
Also, I'm currently dog-sitting for the weekend and haven't been able to get to the Internet for a long period of time because the puppy is rather rambunctious and the only time he's ever calm is when he's sleeping/tired, which is usually when I am sleepy and tired myself from chasing the little thing around and making sure there are no "accidents" in the house. So I might not be able to reply immediately as I usually do because of this responsibility, but I'll do my best whenever I have a free moment.