Ginny Weasley: Traitor?
Ginny is Peter.
No, seriously, hear me out.
People keep making comparisons between Harry&Co. and James generation. That's innevitable to some extent, and I think it's intended. And hasn't JKR said something about someone close to the main characters becoming a traitor?
So, instead of a foursome, the present generation has a trio. And, unless she decides to make one of them go bad (which I hope not), they correspond with Remus, Sirius and James. So it stands to reason the traitor will be whoever fills the "fourth" role for them, and thus corresponds to Peter. But despite fitting a similar niche in the group, it still has to be someone you don't expect -- this is *Rowling* we're talking about.
Neville's a red herring. He's there (at least in part) to make us think, "Oh look, he's just like Peter! He's the traitor!" In the first book, he even plays the tagalong fourth to their group, like Peter's described as doing. But he doesn't really fit that niche, because he stops hanging out with them pretty early on and goes on with his life. Besides, he'd be *way* to obvious.
Then there's Ginny, who *is* a bit of a fourth for them, and definitely tags along, but she got worked in there smoothly enough that we didn't really notice. Plus, we always see her in some specific, stated role (like Ron's sister, or the girl-in-distress, or Harry's potential girlfriend) so we don't really think about how she interacts on a peer/social level like we would with Neville or Peter.
Finally, we don't have much evidence that she's nice. She didn't tell anyone about the diary (yes, she started to, eventually, but she didn't) even though she knew she was probably attacking people. This could be because Tom was constantly sort-of possessing her, in which case she could still be nice, but that's never actually stated. And when Harry rescues her from the chamber, she doesn't ask if anyone's been killed, or if the people who're hurt will be OK, she asks if she'll be in trouble. Which seems to still be her only concern thoughout the whole "reuniting with the Weasleys, wrapping up loose plot threads" scene.
It's probably all a realistic reaction for an 11 year old in that situation, but it's drastically different from how all the other "good guys" in HP behave when they're 11. They all seem to remain altruistic no matter how traumatized they are, which Ginny, interestingly, doesn't. But it's easy to miss that, since she's so entrenched in the innocent damsel role by that point that anything inconsistent with it goes unnoticed.
So. I don't really believe this is what's actually going to happen, but it *could,* and it was fun to write. Mainly, I just hate the damsel-in-distress cliche so much that I'll root for anything that'd turn it on its head -- such as the damsel turning out to be a traitor.
Have fun with slightly wishful theorizing!