If you do know something for sure, please keep it to yourself. This is a "speculation" thread, not a "spoiler" thread.
That said, feel free to discuss anything from the first five books as necessary.
Here's some of the things I think will happen:
Book VI will end with another major character dying. He-Who-Must-Not-Be-Named will have another big fight, though I doubt with Harry. Since he's back in the realm of the living, I think she'll save that for the final book.
A new Minister of Magic will emerge in the end, possibly Dumbledore or maybe Mr Weasley. Cornelius Fudge will be either killed or discredited.
Snape will die a hero, probably defending Harry from one of the Malfoys.
Given Rowling's love of picking up supposedly dangling plot threads, I doubt we've seen the last of Norbert the Dragon.
Either Ron or Hermione will probably die at some point. If Ron dies, Harry will end up with Hermione. Otherwise he'll end up with Cho or Ginny. If neither dies, Ron will end up with Hermione.
I seriously doubt it will happen, but there is the very slight chance that whatever happened to Sirius in that "black hole" didn't actually kill him.
And no question, we'll learn more about Aunt Petunia's knowledge of the wizarding world prior to Harry starting at Hogwarts. (I doubt any of us weren't shocked by the "Remember your last" bombshell from Order of the Phoenix.)