I think Dumbledore is dead, but will help Harry through his portrait in the office. I would find it very difficult to believe if Snape becomes the next headmaster. I would be more willing to believe that part of the plot will be Harry's understand of why Snape did what he did (I do not think Snape is evil - I think he's not a nice man, but he's not evil) and perhaps rescueing him from the prison (can't spell it).
Malfoy still has a part to play. He did not fulfill Voldemort's wishes, but they were carried out, so I think he'll be spared. I wouldn't be surprised if he ends up dying in the last book - although I think it may be in away different than expected.
You know, I agree that these stories are not Tolkien or even great epic novels, but they are well written, very entertaining, and hero is very "boy next door". There is something about these books that just have fired my imagination.