I agree with nOire. It has been pointed out that Gandalf comes back from the dead in LOTR and of course Aslan in the Narnia stories (yeah, yeah, he's a Christ-figure, I got it). Even though the Potterverse is one of magic, there are some things that can't be magically fixed, and death is one of them. I will be supremely disapointed if Dumbledore comes back, even though I really like the character. Besides, now there is only one wizard whom Voldemort fears: Harry. And this gives him the chance to kick some major butt in the last book.

As for Snape, I hope he remains evil. If he is a deeply planted double agent, I just may SPEW.