The Weapons GB.
Another thing we're always doing is concentrating on the weapon platforms, rather than the weapons themselves, so how about a GB where the object is to build a what-if weapon? Eligible items might include missiles, bombs, rockets or guns (everything from small arms up to artillery pieces). The principle is that it should be a single-use device intended to cause damage or destruction to a target, and the element which does the destruction should be un-crewed (a gun is crewed but it's shells arn't), the exception being devices wherein the crew is supposed to die in the process, i.e. a suicide weapon like an Ohka.
The question arises: to what degree should launch vehicles be included in the build, i.e. if you build an anti-aircraft missile, can you build the launch vehicle it travels on? I would say yes, if the launch vehicle is single-purpose. That is to say, it has no purpose other than carrying and launching the weapons. Thus a tank chassis used as a SAM launcher would count, but a fighter plane carrying an ASM wouldn't, since the fighter plane can do other things too.