The way she has been writing the books it will be someone we don't expect.
I didn't expect her to kill Cedric in the 4th book. As for Sirious, unfortunately prior to me reading the 5th book, POA came out and I read an article in Entertainment Weekly and well Gary Oldman thought he should express how "upset he was that his character died in the 5th book". So Gary Oldman ruined the 5th book for me prior to me getting to read it.
I think Dumbledore and Hagrid are just to obvious. I also think the same thing goes for Ron and Hermoine.
Now we are assuming it is a "main" character, when in fact all she said was that she was planning to kill a character off. For all we know it could be another person in the Order of Phoenix (like Lupin or something).
I personally think it will be some random character, someone whom she has mentioned in at least the fourth and fifth book and who is kind of important but not really.
Hey maybe it will be Malfoy. Oh wait we need the conflict. But wait the books will still have conflict without Malfoy around. So my vote is Malfoy.
