What about killing off:
Peter Pettigrew
Another vote for Dumbledore…
How many books are left in the series? Two or three?
Predictions:
Rowling will create a power vacuum in the wizarding world with the death of Dumbledore. The Ministry of Magic will attempt once again to take over Hogwarts. Someone like Remus Lupus will temporarily be named the Headmaster of Hogwarts, but the battle to head off Voldemort will fall on Harry Potter’s shoulders & Harry Potter’s alone.
I see Hagrid as being integral to the “heart” of the series. I don’t think Rowling would kill him off until the last book, if ever. He is the one that guided Harry into the wizarding world. In many ways, he stood as a father figure to Harry. One of Harry’s guides, Dumbledore, McGonagall, Hagrid, must mentor him to the end, just not be there at the critical showdown with He Who Must Not Be Named. 
Definitely mysteries about Severus Snape will be revealed in the next book or so. He will have to be unmasked as a double agent at some point. His mettle, loyalty, etc. will be tested. Hopefully, he & Harry can reach some sort of understanding.
I’ve been hoping that the spirit of Sirius Black will come back to help Harry or that he will be invoked at some point in the plot.
HP & the Goblet of Fire is still my fav, so far, from the series. So grand in scope & execution, with the gamesmanship lessons and tragedy & grief issues.