Here are some theories on the 7th book that I am very, very convinced about...I'm wondering if anyone else has thought of them?
1. What Dumbledore experiences in the cave/his worst fear/what his boggart would be is seeing someone (or multiple someones) he cares about hurt. This someone (or someones) will be a completely new person who has never, ever appeared in the books.
2. There will be a new DADA teacher (according to JKR), which suggests that Hogwarts will stay open.
3. Petunia is acquainted with Dumbledore in some way besides letter-writing. I think she may have been involved with the order or else met him in person in some other circumstance.
4. Snape will die. Voldemort will die. Harry
might die.
5. Hermione will stay to teach at Hogwarts.
6. Dumbledore is very insistent that he trusts Snape, so here are two possibilities for why he is so certain:
a. Snape has been loved by/loved Lily and is genuinely distressed by what happens to her
b. Snape made an Unbreakable Vow with Dumbledore regarding his alleigance, as well as making one to carry out "the plan" regardless of his personal feelings.
The other possibility is that Dumbledore has been wrong all along and Snape is genuinely evil, but I don't believe this is the case.
7. McGonagall knows very important things about The Boy Who Became Lord Voldemort.