Now, bear in mind that I haven't even had a chance to reread book six since then. If you want to debate over details and hints, that's cool but don't expect me to cite things. :)
There's no book seven spoilers here (except for one note from JK's public comments) since I haven't seen any advance info on it other than size (about 760 pages) and weight (about 1.8 pounds).
(New notes are in italics)
Guess: Dudley Dursley, and possibly Petunia Dursley, will be reconciled with Harry. If it's Dudley, he will have some hidden power or magical connection. They will provide a key piece of information or timely assitance to aid Harry.
My wife tells me that JK's said not to expect anything from Dudley or much from the rest of the Dursleys. I'd still like to see Petunia come around. I'll predict we'll at least get a scene where Petunia's past (and relation with Dumbledore) is resolved.
Guess: Grawp will take action at some pivotal moment, dimly aware of what he's doing. The dark forces will not expect this because they don't consider him an actor, being non-human.
We saw Grawp mentioned periodically throughout this book though he had no bearing on the story. Thus, I expect him to be (momentarily) significant in the final book. I'll go on a hunch and say that he will witness some confrontation and step in against a dark wizard he thinks is fighting underhanded or to step in and save Harry or Hermione during some moment of physical peril (the proverbial cliff-hanger).
Guess: Dumbledore died at the end of book six. But he will return, in a different form, to guide Harry in book seven.
I'm told there's some discussion about whether he actually really truly died. I think he did. But, following the structure of the myth, he will still aid Harry in a time of need. Since we're in a magical world and we've seen numerous ways to interact with dead characters, I think his return will be more than just Harry recalling sage advice.
If I had to pick something, I'd guess a painting. Kit's leaning towards Fawkes, which I don't really consider that "Dumbledore" but that doesn't mean it won't happen, too.
Guess: Harry will kill Voldemort. Snape, having evaded Harry, will be there to help at the critical moment and finally prove that he is on the side of good.
Voldemort dies would seem to be a "Duh!" given everything we've seen so far. Snape is really in a bad spot at the end of book six. I think he killed Dumbledore on his orders (just as Dumbledore made Harry swear to follow his commands, no matter how extreme) and will use this to get as close as possible to Voldemort. Harry will think he's fighting both of them.
Snape will backstab Voldemort at a key moment. But will Snape die in the process? I could see it working either way. Since these are supposed to be predictions, I'll pick "no, but it'll be close."
Too Cool To Happen: Harry discovers that he is one of the unknown Horcruxes used by Voldemort. He kills himself at the same time that he kills Voldemort.
Weasley's Wizard Wheezes got a lot of mention in book six. (From genhusky's comments) They and their products will be leveraged in book seven's battles.
Will Luna make any unique contribution to the final confrontation? Perhaps some information or contacts? (From twopiearr) Neville will trust her and act on her information at a key moment.
Was it just me or was Trelawny left hanging in book six? Last time I recall her being mentioned, Harry had left her in the 7th floor hallway. Odd for Rowling to not give some mention at the end. (Was she at the funeral?)
I'm not looking for this to be a key point, I'd just like to see some sort of conclusion for her. (kai_ta_loipa posited that the Death Eaters might've kidnapped her in the confusion. Interesting idea, though I don't know that JK will go that direction.)