for a pirate vs ninja competition you could have then do a contest like a photo scavenger contest. The teams would be given the assignment of getting photos throughout the con and they would turn in their pictures to be judged before the dance off for an extra points or a different award. You could even get different pictures for both groups.
Ninjas for example could have, best assassination, most photo graphic ninja, or most successful stealth maneuver. While the pirates have, best high seas duel, scurviest pirate, or treasure chest. There would of course be some ground rules about appropriate pictures, and getting people to participate.
We tried this once at a Halloween party and it was great, the only real requirement for the teams is that someone should have a camera (preferable digital). Of course judges would review the pictures and award points for creativity, originality, fitting the category, and general look of the picture.
What do you think?