I don’t think Griffindor’s sword could possibly have been a Horcrux without Dumbledore knowing it . . . but I do like the idea of the four founder’s artifacts each corresponding to a tarot suit. Swords, wands, cups and pentacles (amulets) . . . we already know about Slytherin’s locket and Hufflepuff’s cup, so that would suggest that the mystery Horcrux is Ravenclaw’s wand. Which (assuming Dumbledore is right about the others) would mean the six Horcruxes are:
Tom Riddle’s diary
Marvolo Gaunt’s ring
Slytherin’s locket
Hufflepuff’s cup
Ravenclaw’s wand
the snake Nagini
Also I’m convinced that R.A.B. is Regulus Black, and I’m guessing the clue to the locket’s whereabouts (if not the locket itself) is in number 12 Grimmauld place. I wouldn’t be surprised to find out that the locket had been there, but was sold by Mondungus Fletcher.
As far as other predictions go, I don’t think Harry, Ron, or Hermione will die. I’m operating on the theory that J. K. Rowling wants to give us a happy ending, and I think killing any of the big three would make that impossible. In fact, I’m guessing Ginny, Fred and George survive as well – killing any of them would be needlessly cruel. The older Weasleys I’m not so sure about, but if Percy survives then I think he will reconcile with his parents. I take the prophecy literally, so that means Voldemort will die if Harry lives. I think Snape will sacrifice himself, proving he’s on the right side in the end. I’d bet on Wormtail dying too, but not until he repays his life debt to Harry. If someone close to Harry dies, I’d guess either Hagrid or Lupin, probably Lupin. I just have a feeling all the Marauders will be gone at the end of the book.
I’m guessing Harry eventually becomes an auror or will be well on his way to doing so. I also think Ron and Hermione will end up together, and so will Harry and Ginny. If I remember right, Rowling hinted that one of the students would become a teacher; I’m guessing Neville takes over teaching herbology. She also said someone would perform magic late in life, but denied that it was Aunt Petunia, so I’m guessing Filtch.
The biggest mysteries to me are: Why is it significant that Harry has his mothers eyes? (Rowling has said it is.) Why did Rowling say (in an interview) that there’s more to Aunt Petunia than meets the eye? Why did Dumbledore have a “gleam of triumph” in his eyes when he heard that Voldemort had restored himself with Harry’s blood? I don’t have satisfying answers to any of these.
One somewhat out there theory I’ve read and kind of like is that Dumbledore is related to the Weasley’s, and that all of them are descended from Godric Griffindor.