I'm a bit worried about book 7...it feels like Rowling is rushing thigns abit now, after book 5....it would be a real shame if she wrecked the whole series by putting some completely twist at the end, or just went for a tragedy (everyone dead except Harry). Mind you, if she went for a very happy ending, then that would feel wrong too.
I think the best ending would be if most people survive, but either Ron or Hermione die in the final battle. Or...even better...Harry dies in the final battle in order to stop Voldemort. Remember, they have that link? That could be a vital part of the plot...after all, why else did Rowling make sure to slip it into the series so early, hmm?