I suspect that one of the characters will be Snape. Frankly, given what he did in the 6th book, I don't see any other way out. If it turns out he really did betray Dumbledore, then that means Dumbledore was wrong and Harry right from the very first book -- which makes no sense, as Dumbledore is always right. So, Snape has to be a good guy. However, since he's now walked into a figurative lion's den, he's probably not going to make it out alive. I suspect a heroic sacrifice to finally redeem himself in Harry's eyes and pay back whatever debt he feels he owes to Harry's mother from their own schooldays.
As for the other, it's a tough call. I thought maybe Draco, but his archetype seems to me to be more the redeemed traitor -- rather like Snape, but his redemption hasn't yet come. So, to have Draco redeem himself and then die would be fairly cheap. Thus, I doubt it's Draco.
I'm also ruling out the remaining teachers -- I mean, really, why kill them? So, it's really down to our two remaining central figures: Hermione and Ron. I kind of like the dramatic symmetry of the best friend sacrificing himself to save his friends, so, I'm voting for Ron as number two.