This is what I think will happen in the Book 6 (mostly) and some might carry over into Book 7...It's kinda long, sorry...But I think it brings up some good points! Let me know what you think!
I think the half blood prince is Mark Evans mentioned in OotP, and has some relation to Harry through his mother/Aunt Petunia. He would be Harry's cousin and perhaps Mark's parents were killed in the war against Voldemort and his followers as well, which would explain why the Dursley's are Harry's only remaining living relatives he could stay with. Maybe Aunt Petunia comes from a wizarding family and she is a squib like Filch and therefore hates magic b/c she can't do it. It's kind of odd that Uncle Vernon always talks about Aunt Marge yet we don't know anyone except Lily (Evans) Potter from Petunia's side, and she's dead...Or maybe, Lily and Petunia had a brother who fell in love w/ one of Lily's friends from Hogwarts and produced the half blooded Mark Evans...It will be interesting to see which house Mark ends up in. Gryffindor or Slytherin. Maybe he will be the Half Blood Prince of Darkness. It could get pretty interesting if Harry has to battle against a cousin other than Dudley. But then again, I doubt Lily would have a friend in Slytherin who would produce the Half Blood Prince that would join Voldemort's side...Mark will probably join the DA and fight w/ Harry against Voldemort....two half bloods against one...
I think Dumbledore's Army will gain new members. And there will be another battle w/ the Death Eaters...Maybe Voldemort will show up and face Harry again. The DA will work closely w/ the Order, and I think some people will die on both sides...
I think the veil in the Ministry holds a secret (obviously). I think everyone killed by the unforgivable curses are behind the veil and that Harry will use the two-way mirror to talk to Sirius and possibly even his parents. Where did Sirius get that two-way mirror anyway? I hope he brought it w/ him when he went to the Ministry!
I think that Arthur Weasley will become the Minister of Magic. A lot of pure blood wizarding families (i.e. the Malfoys) won't like that though. Since Lucius is unable to act since he is locked up in Azkaban I think that Narcissa and Bellatrix will begin another wizarding war against the Ministry in the name of Voldemort.
I think Hermione and Ron are going to dance around the fact that they actually like each other and do some more flirting and whatnot for a good portion of the book and then kiss. They will then be forced to decide whether they actually do want to be together now that it's out in the open. Maybe Ron will have a moment and actually bear his soul and tell her of his hatred for his ex-hero of the Quidditch Pitch, Victor Krum, and finally tell Hermione how he feels... Although, if they do get together, they're going to have to get married after school, b/c there is no way they can be together and then not....unless she ends up w/ Harry (which I don't think will happen b/c I think Harry will know how much Ron likes her--think the Dawson-Joey-Pacey love triangle from Dawson's Creek and how Joey ended up w/ the side-kick, not the title character)
I think the Weasley twins will come back to Hogwarts. Percy will still refuse direct communication w/ his parents. Ginny will get together w/ Neville. Maybe Percy will die. In the 5th book I thought it was possible that Percy was working as a double agent for the Order against the Administration at the Ministry, but I would have thought that that would have come out in the conclusion. I don't think JKR would have let that go into the 6th book w/o explaining his role...
Something big is going to happen w/ Snape too, I can just feel it. We wouldn't have learned so much about his youth in Book 5 if JKR wasn't just setting us up for Snape's secret. Maybe it will be what he did to make Dumbledore trust him. I think in Book 7 Snape will die to save Harry. But before he dies I think they will have an understanding, and Harry will have a respect for Snape that he never thought possible. I think that Harry will inherit Grimmauld Place from Sirius and he will need a guardian in the wizarding world. B/c of the fact that he is sometimes possessed or whatever by Voldemort, I don't think it will be Dumbledore. Mrs. Weasley always said Harry was like a son to her, but that just wouldn't be as entertaining as Snape watching over him in Grimmauld place. I think it is a rather far-fetched idea, but it would be fun to watch Harry squirm under the ever-watchful eye of Severus Snape 24/7.
Dumbledore will probably have another reason to be distant again. Probably b/c he thinks that Harry can handle it, after all it will be the 7th time he's gone up against evil and triumphed (including when he was a baby). I'm sure JKR will have an awesome conclusion which brings everything together and makes you go "WHOAH!"as per usual.
I read something that JKR said about this will be Harry's shortest stay at Privet Drive yet. So perhaps something will happen early on and he will go to the Burrow or to Grimmauld Place. Maybe Harry will bring Buckbeak back to Hogwarts, that would make Hagrid happy and Draco really angry! But w/ Arthur as Minister of Magic and Lucius in Azkaban, I don't see how that would be totally implausible. I also read that Grawp becomes more tame. Maybe he and Hagrid go back to the giants along w/ Madame Maxime and convince them to come on the good side, or at least some of them. I'm not sure the giants will play a huge role in book 6, probably more in book 7 for the final battle.
I think Harry will continue in his studies to become an Auror. I wonder if they are going to have a decent Defense Against the Dark Arts teacher this year...It would be about time, Lupin was the only good one they had ever had...closely followed I'd say by the Imposter-Moody. I think Snape will be the DADA teacher in their 7th year...he'll finally get his wish and then he'll die...Professor McGonagall will continue to be the mother hen, watching out for her Gryffindors. And she will continue to help Harry on his way to become an Auror.
That's all I can think of for right now...later