Sorry to be an anorack - but they aren't dead - they are insane and committed (having been tortured over an extended period by Barty Crouch & Co with the Cruciatus curse - I believe)
Edit: Or did they die in the last one while I as nodding off?
No, you're right. I thought they were killed off because of torture, but they are just infirmed. I only remembered that Neville's grandmother was his guardian now and assumed that it implied the death of Neville's parents.
Still, I do believe Neville would make a sacrificiable character that wouldn't put permanent damage into the whole narrative continuum, especially if there were ever to be an adult Harry Potter.
I objected to the prediction of Hermione's death because people are still curious to see how powerful she'll become. The future of women's lib is tied up with her. As well, there's the prurient curiosity as to whom she'll marry one day: Will she and Harry just be "friends" for life, will Ron haev enough courage to express his true interest in her, will Hermione sacrifice her sexuality for her career, or will Hermione try to be superwoman by having a family and career and balance them perfectly? The history of modern womankind is invested in her.
Harry Potter could be sacrificied, perhaps, if you want to turn him into a Christ-like figure.
I could also see Snape sacrificing himself as a form of penance to Dumbledore and personal redemption for himself. Snape has no children and no wife and has led a complex life. In the Armageddon of magical duels, you could eliminate him and still have a story that resolves harmoniously. Just IMHO