"'None of us could've guessed Snape would ... you know,' said Ron."
That's the beauty of Rowling: we can toss ideas around about what will happen in Book 7; some people have even read all six published volumes with fine-toothed combs looking for hints and clues; many theories have been developed (some of which might be right in parts)—but when it comes to down to it, we can pretty much count on one thing from JKR: she will surprise us.
I have now finished rereading all six volumes, and I am ready for July 21. Perhaps chapter one will be about Bill and Fleur's wedding ... on July 21? :-)