i just watched this freerunning competition on tv. if you dont know what free running is you should google it because its actually pretty cool.
my challenge suggestion is have some kind of course and have a rigged character availible. the challenge would be a timed session. the judging categories would be on reaslism style and overall "coolness" of the animation. i think this would be a fun way to use bone animation. everyone would be in the same direction but would have complete freedom over it.
Do you mean everyone gets the same character and the same course (as a download) and it will be pure about the animation.
If so, it could get real interesting, especially when it's really only about animating and even the boning is done and it pure and only about the animation.