"What do you think the special announcement could be about?"
Honestly, it's kind of a no-brainer. It has to be about Kingdom Hearts 3. If not that, then some sort of Kingdom Hearts anime/movie similar to Advent Children. I can't imagine it being about anything else since it's considered "special". I doubt it's any kind of April Fool's Joke.
I have yet to read the Insider forum response, but it wouldn't surprise me if they had similar theories.