So here follows my ramblings on the subject. I'm sorry if I've forgotten any characters/messed up events from past books/written sentences that don't make sense but I'm tired and need to go to bed. I just wanted to get this done before tomorrow so it would be before the book came out and people started talking about it all over the internet.
Characters I think are definitely safe: Harry, Hermione, Ron, Draco.
Dumbledore - He is basically all that is good and right in Harry's world. Killing him would fit in with the idea that everything has to get as bad as possible before it gets better. And he's the mentor and fountain of all knowledge so I suppose it would be akin to Obi-Wan Kenobi's death in Star Wars. Harry needs him, and without him he'll have to stand alone. But still, I don't see it happening for that very reason. Kill Dumbledore and you kill hope and goodness.
But maybe that's exactly what she's going to do.
A Weasley - This would seem to make sense. Harry is very close to the Weasleys and practically a part of the family himself so one of them dying would hit him hard, plus there's so many of them it's like JK was giving us surplus so she'd have a few to kill off if need be. But who, if any? She's already killed off Sirius, a well loved adult and guardian, so one could argue that Molly and Arthur are safe since it's awfully soon to kill another.
Bill or Charlie could die but while this would devastate the Weasleys themselves it probably wouldn't have much emotional impact on the reader since we don't know much about them. Fred and George are the comic relief, killing them would be counter-productive. Ron is protected by being in the main trio and I think Ginny has more of a story to grow in to. She's the only person to have encountered Voldemort on such an intimate level and I feel her character has further to go.
This just leaves Percy. He has to return to the good side some time, it's inevitable. The question is, how? His betrayal hurt his family deeply and how is he going to make up for that? Yes he could just come back but that makes the whole storyline a bit pointless, there must be something he's going to do. I think he's going to realise his family were right, join the Order and use his position to help them. This has the potential to put him in danger and possibly sacrifice himself for the cause. If any Weasley's is going to die I would say it'll be Percy. The ultimate act of redemption.
Neville - A possibility, but again I think he has further to go. In the first books he was a bumbling, useless side character and it's only recently we've found out he's actually much more than that. I think he's going to go on to do great things. He's a hero.
Snape - But then who would torment Harry by abusing his position of authority?
McGonagall - Hmm, maybe. She's the typical 'stern but fair' teacher that was so popular in all the early 20th century books I read about girls at boarding school. She's a good guy, well respected and pretty well liked by Harry and co. Her death would certainly shake things up at the school and in the story. She's been a constant figure from the very beginning and her loss would have a profound effect on everyone. And to be honest I can't think of any compelling reason why she shouldn't die.
Remus - In some ways an obvious choice. Friend of Harry's, important in the Order, a popular character, very similar to Sirius (regarding place in the story, not character.) But then it's that last one that would seem to argue against him. Having already killed Sirius, killing Remus seems redundant.
Hagrid - He's been a good friend and support to Harry since the beginning. Killing him would really upset the trio's world. But then what would happen to Grawp? For that reason I think Hagrid has to stay. Unless JK was bringing in Grawp to fill space Hagrid's death will leave. We know that he still has to develop as a character, I just hope that doesn't come at Hagrid's expense.
Lavendar, Parvati, Seamus, Dean - They've constantly been there in the background. Fellow Gryffindors, acquaintances of the trio, pretty well known to the reader. Are they important enough for their deaths to be a big thing? I'm going to go with no. Yes, it would be horrible for the characters who live and go to school with them but as readers we're not really that emotionally involved with any of them. I think the only way any of their death's would be important would be if they were killed at Hogwarts because that would show how even the school isn't a safe place any more.
Luna Lovegood - She's only just arrived on the scene but already she's become quite an important character, immediately joining Harry's circle of friends. But for that reason I don't think she'll be the one to die. It's too soon and setting up a character just to have her killed seems lazy. JK likes to rip our hearts out.
Tonks - Again, she's only just been introduced. It seems far too early to kill her.
Crabbe or Goyle - Eh, who would really care?
Cho Chang - Will we be that lucky? I doubt it.
Sirius - Yeah, he's already dead but come on. He fell through a curtain in the Department of Mysteries people! Obviously his storyline isn't finished, but to be honest I doubt killing him again would be what JK was referring to.
So, my top three picks for the death are: Percy, McGonagall, Hagrid.
Maybe Dumbledore. I keep wavering on him. Maybe he'll die and McGonagall will take his place.
I really should stop this and go to bed.