I think that Neville will play a big role in books 6 and 7. After all, he is in Gryffindor. If he was going to insignificant (on the whole) to the story, I think that he would have been placed in Hufflepuff. Also, she would bring up how his parents were such great Aurors. I think that there's a lot of great wizarding ability in Neville, he's just kinda been intimidated by his Gran.
As far as who the HBP is, I have no idea. She might introduce a new character. That almost seems more likely to me, because, as far as I can remember, she hasn't really stressed the backgrounds of the other half-blood boys.
I absolutely cannot wait for this book, but I know that I'll read it within 2 days of its release (just like I did with OotP), and then be on edge for the 7th book.
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