Don't diss Neville! He's like, uber!
I think that Dobby is a strong possibility. You'll notice that most of the characters that die happen to be Harrys protectors (mum, dad, godfather, dumbledore...). Yes, I know, killing off Dobby is like mashing a Kitten to death with a plastic spoon, but if it happened to Dumbledore it could happen to anyone.
My money, however, is riding on the fact that the entire story is an April fools joke.