It was Chamber of Secrets that was originally to be Half-Blood Pudding. A quote from JKR:

I was delighted to see that a hard core of super-bright fans knew that the real title was once, in the long distant past, a possibility for 'Chamber of Secrets', and from that deduced that it was genuine. Certain crucial pieces of information in book six were originally planned for 'Chamber of Secrets', but very early on (first draft of Chamber) I realised that this information's proper home was book six. I have said before now that 'Chamber' holds some very important clues to the ultimate end of the series. Not as many as six, obviously, but there is a link.

Which leads me to think it's Snape.

On edit: I was thinking PoA when reading CoS. Now that I'm mentally in the right book, I'm guessing it's gotta be Tom Riddle.

Which leads me to think it's Snape

Iiiiiieeee! I like that idea. I like it a lot.

I'm guessing it's gotta be Tom Riddle.

And that she was being tricksy by saying it wasn't Voldie?

He renounces EVIL and takes back the Riddle name?

He becomes the Riddler!

He uses the powers within his magic diary to edit Ms. Rowling?

Loving the Potter speculation.

Also, from a while back:

In other news, Haruki Murakami's After the Quake, a collection of stories about the aftermath of the 1995 Kobe earthquake which killed 4,000 people, is just marvelous ... The stories are really hard to describe--not slow but very careful, lucid, all these details and moments building up to something extraordinary at the end. Quiet moments of connection or disconnection, hope or despair, discovery, humor or joy or sorrow.

I freaking love Murakami. Love love love love love. After the Quake is a wonderful collection, and I think you describe the feel of the stories really well. The insanity behind the ordinary. Have you read any of his other work? The Elephant Vanishes is another collection of short stories, although they don't have a unifying element like those in After the Quake. And his novels are exquisite, he absorbs you into a parallel universe of perfect sentences.

He uses the powers within his magic diary to edit Ms. Rowling?

BWAH!

I've read some of the stories in The Elephant Vanishes (I discovered him via the reprinting of "The Second Bakery Attack" in The Year's Best Fantasy and Horror, which is a story that just fills me with giddy joy whenever I reread it), but none of his novels.

For Susan, especially, interesting article on historical novels in the Globe and Mail.