Feel free to include your predictions here, but please note, this excludes stats (since we may not be getting them), but isn't limited to: unrevealed character relations, bio (blood type, age, weight, height, eye&hair colors), chakra natures, technique "physics", concepts and so on. Pretty much any contents it may have and I'd also like to hear how you would feel if your personal prediction or theory were to be proven wrong by the book (I'm looking at you sand&magnet release).
Mod edit: Along with theories, this thread also includes some information from the previews of the Fourth Databook.