My first thought, or hope rather, was that it would be the announcement of a release date for the much anticipated encyclopedia. I'm sure I'm wrong though.
Most people seem to think that it could be a huge Harry Potter based online role-playing game, which would mean that my nerdiness is going to skyrocket, something most people wouldn't deem possible. It could also very well be a regular website, an announcement of a new textbook along the lines of Fantastic Beasts and Where to Find Them, something charitable because we all know Rowling loves her charity work, or even a personal statement declaring me as her number one fan. Probably not the last one.
Let the 5 day waiting commence!