Now that I've reread books 1-6, here are my predictions for Harry Potter book 7.
(Spoilers for books 1-6 only; everything about book 7 is pure speculation.)
1. R.A.B. is Regulus Black (duh). Going out on a limb, maybe he's Crookshanks. (Sirius knew, but didn't tell Harry.)
2. Percy is not merely an asshole but also working for Voldemort. (Suggestions of this: It is mentioned several times in the first three books, before we know who Scabbers is, that Ron got Scabbers as a hand-me-down from Percy. How did Percy get Scabbers? Also, the whole Barty Crouch coverup is sketchy.)
3. Wormtail betrays Voldemort at a critical moment, Return of the Jedi-style.
4. Neville and Luna, sitting in a tree. (And obviously Ron and Hermione too.)
5. Bill Weasley dies valiantly.
6. The voices that Harry hears behind the veil in the Department of Mysteries are the eponymous "deathly hallows".
7. The giants will become involved in the conflict. Harry will wish he had paid more attention to Professor Binns's endless lectures about the giant wars, but Hermione will save the day by knowing key information.
8. Dobby provides Harry with information about the Malfoys.
9. After seeing the effects of the relationship between Harry's and Voldemort's wands at the end of book 4, Voldemort had Ollivander kidnapped to get more information (possibly using Veritaserum) about this wand relationship that he can exploit.
10. Zacharias Smith is a descendant of Hepzibah Smith (the old witch whom Voldemort killed for Hufflepuff's cup and Slytherin's locket) and therefore of Helga Hufflepuff.
11. Hogwarts doesn't close, and Harry goes back at least once.
12. Ron Weasley ends up asphyxiated on the moon. (Just kidding!)