I'm not really familiar with the whole "challenge" process, but a think a good one would be to create a passable version of one of the world's most exotic weapons, the hunga munga, or the African throwing knife-dagger.

I'm pretty sure you can't make this throwing, but whoever can make something like this would be pretty awesome.

A guy who practices here named Ogg made throwing axes by making whites that're just foam and cloth.

hmmmm.....