Hum. I like the Snape Dark Arts suggestion. However, I think that's a major character development that might be better off in book Seven.
I, like Dan, thought that the metamorph character was a HUGE gun on the mantle. I kept expecting one of the characters to turn out to have been her in disguise all the time. It's still too much of an opportunity for a Rolling-esque surprise for her to ignore, I think. I wouldn't be surprised to find a 'new' student in the next book who turns out to be Tonks.
As for the Harry/Tonks thing, I think it's a bit of a streatch, but not an unbelievable one. Personally, I think he has to end up with a student we've been following for a long time for empathy. I don't think Rolling would pair him off with an older woman simply because that might seem like too much of a betrayal to her young audience, who would subconciously wonder why one of the students wasn't good enough.
So, I predict a new Dark Arts teacher for the next book, Snape in the seventh. Tonks pretending to be a student, or at least someone unexpected, in the next book. (Which, actually, could let SE's Tonks relationship have an interesting little twist...) And Harry ending up with a student. (Ron's sister? Yeah, I know, seems too obvious.)
Now, here's another question. You think she's planning another series after this one, Harry the Auror? You'll notice the beginning, which lists previous books, lists them as 'First year at Hogwarts' 'Second year at Hogwarts' ect. I wouldn't be surprised if she had Harry end up with someone simple at the end of this series, say Cho, to give some completion, then had him go on for marriage to someone else later on.
