and Gossip Girl will turn out to be...
Dan Humphrey again, doing research for a sequel
Or, maybe as a throwback to a different theory, Dorota. It'd be great, she could come out of nowhere, saying she was bored and wanted to teach a lesson or something
Just hope it's better planned and executed than Dan was (I've always hated that he was a great character to the choice, it's just clear that no real planning went into it; unless he had the posts timed so he could be publicly present for them)