wow, so many theories.
here are mine re the characters:
Harry - Will be a horcrux - explains why voldemort nearly died trying to kill harry: he put the second last 7th of his soul into harry and only had one 7th left. It was such a shock to the system he nearly died - remember all the stuff slughorn said in the memory about tearing the soul and how no one had ever done it before - 7 pieces is a lot and that last attempted tear was obviously particularly severe. He did kill (lily) which was the tear, and made Harry the horcrux, unknowingly of course. I think harry will discover this, perhaps only in battle with Voldemort, or not long before, and will sacrifice himself to ruin voldemort. I have been reading all the potter stuff in newspapers this week and one article made references to the Hero thing, and how they never survive and there is always sacrifice involved. They aren't made to live a normal life like everyone else (as has been focused on in these books so far) and ultimatly end up sacrificing themselves so that the rest of the world may live in peace...touching really.
Voldemort - obviously he has to die. It is a childrens/adolescent book after all.
Ginny - maybe she will die...not too sure, but i don't think she will just let harry go off on some dangerous journey without her. Perhaps she will die to try and save him.
Ron - will live methinks. But he will do something good. Rowling is a talented writer, which means nothing is in the books just for the sake of it. She wouldn't have written Ron's character in if he didn't have a big role to play, and so far he hasn't really. She has said she knew how the whole series ended before she started writing them - Ron has been Harry's best friend for the whole series for a greater purpose than just to highlight the difference between HArry and Draco, and support the 'good' side.
Hermione - will live, personally i can't see her killing off either Ron or Hermione. I think if she kills harry, she will keep these two alive to champion Harry's cause and remember him.
Draco - i also have read fanfic and desperatly want Draco to appear good in the end. Perhaps, as in one of the more memorable fanifics i have read, he will become good, somehow join Harry, Ron, Hermione and Ginny and battle with them against Voldemort. Maybe not. I can see him killing Lucius. Seems plausible for me that Lucius gets out of Azkaban and is fighting beside the Dark Lord when Draco takes revenge for the years of mistreatment and takes Lucius for himself to kill.
Snape - not sure. Somehow i think if Draco becomes good, snape won't. People (well, most people) want him to be bad; they want to hate him for killing dumbledore, and whilst there are many plots about how it was dumbledores idea for snape to kill him, it all seems a bit, to use your word, fanon to me. I can't forget the bit where rowling purposely points out that Harry heard fear and pleading in dumbledores voice for the first time the night he died and was begging Snape not to kill him. Maybe he was begging him to kill him i suppose people would say, but it doesn't seem plausible to me.
Then again, i can't see snape fighting alongside Voldemort either, so maybe he will turn good. THe thing with Lily seems plausible to me.
Lily, James, Sirius - something said right at the start of this thread about Deathly Hollows gave me the idea that maybe they come back through the veil, a sort of ghostly army that Harry can command. It said something about how it is the closeness between life and death - a portal through which dead souls can return to the world or something.
Dumbledore could possibly return as well, but i don't think so.
Then again, when Harry says at the end of HBP about never getting Dumbledores advice again, he failed to remember the portrait that rowling pointed out as now hanging in the headmasters office - could that not give him advice, as dippet etc. did to dumbledore?
Blaise - again, Rowling doesn't include things in her books unless they have a purpose. Why bring him in in the 6th book just to be in Slughorn's club, but make him slightly more central than some of the other characters in the club. Maybe this is fanfic influencing me again, but somehow i think he has something to do with the last book.
Luna - Dunno. She will be there with them i suppose, and may do good deeds, but i don't see her as important enough to be killed.
Neville - i think he will die. When you think back through all the books, HArry is the only person really (and his friends) who ever did anything for Neville. THe DA gave neville something to look forward to etc etc. Harry is his hero. I think he will die for him. (am i being a bit romantic/dreamy thinking up all these emotional scenes with sacrifice being a central theme??)
Order of the Phoenix in general - dunno. I suppose some will have to die. But i don't think Lupin will. Maybe Tonks though. Otherwise, we don't really know any of the others enough for them to matter a hell of a lot.
As for other characters like wormtail, the rest of the weasly's etc. Not enough emotional significance for them to die, or for it to be a major part of the story. I'm sure Rowling wants people to be bawling their eyes out over this book, but also have a little hapinness/hope at the end, and she won't make it a complete bloodbath, so minor characters will probably live i would assume, mostly.
Yeah. Can't think of much else. But i'm sure i will before it comes out.