I don't remember the kids saying "Oi!" quite so damn much in previous books, do you? Therefore, my prediction for Book 8 (once Rowling succumbs to loneliness without her characters to keep her warm at night, and to the pleadings of Scholastic to come and bail them out of bankruptcy) is that Voldemort's soul shattered once again and fled to lodge in Donna, the really annoying not-Companion from the Doctor Who series 2 Christmas special.
(Oh, come on. The sonic screwdriver is so a wand. So are the Dalek face-suckers. There's no way crossover fic hasn't been done on this one. No, I'm not writing it. I know a few of you do fanfic, so be my guest.)