Valve have just put up a new chapter of the ongoing comic saga that's been accompanying the more recent class updates. And it's about the Engineer, the only class who hasn't had an update yet. Well, I'm out of theories.
The comic does shed some light on the recently discovered Golden Wrench , though. It explains that TF2's radically advanced technology stems from a gold-like mineral called Australium. The end of the comic suggests this material is also the source for a new set of weapons the Engy is about to build, so an Australium Wrench would explain a lot.