I think it's going to be Arthur Weasley. JKR obviously loves him, and he's an Unreliable Male Role Model. Someone over in ataniell93's journal mentioned that the slightly cracked patriarch was a stock character for school romps. They have precious little actual character development, and are generally only of use as pawns, and pawns are sacrifices.
The death of Arthur Weasley would bring a straying Percy back to the fold, might sober up Ron, and would turn the twins from merry pranksters into avenging angels. Molly would be devastated, of course, but her family would rally around her.
Ron would come to be closer to Harry; Ron's perfect family would no longer be so picture-book whole and complete, and Ron would understand where Harry was coming from. Despite the Weasleys wholeheartedly embracing Harry, Harry's always an outsider.
It would serve a narrative purpose, I think, and I don't see Rowling as someone who kills off characters without too much of a narrative purpose. Trelawney would be no big loss. Snape would be a devastating loss for the Order of the Phoenix, but Harry wouldn't miss him too much, and some readers would be sighing relief. Snape's going to become more of a sympathetic character in the next book, I think -- I entirely think that Harry's discovery of his father's gang bullying Snape was only the start of an interesting dynamic that's going to wind up being crucial in the final book. I don't see Snape dying until he's needed the most. I don't think Dumbledore will die for a good long while either, he being Voldemort's opposite number. Unless Hagrid starts becoming more important, I don't see him dying.
I don't see Harry's schoolmates starting to drop like flies either. Cedric was a bit of a fluke, I think, just to prove to Harry that yes, he's mortal.
I don't know. I'm rambling. But I still think it's going to be Arthur Weasley.