I'd love to do something similar to that scene in "First Knight", where the bad guys show up and try to force King Arthur to surrender. He announces to the crowd "My last act as king is to order all my subjects to...FIGHT!!" and it's on like donkey kong from there.
I'd get a cast of my original characters as the good guys, and come up with some suitably creepy villains. (My friend showed me some stuff from his role playing material- the Cyber Papacy, these creepy theocrats who sound like they would be right at home in the Warhammer 40k universe)
So we'd have a huge cast of good guys and villains duking it out, but not just on stage. Some of them would be planted around the room, and they would rush up to join the action too. In the end, the good guys win..I would change the ending a bit- turns out our stand-in for King Arthur was wearing a bullet proof vest under his clothes- imagine the look on the bad guy's face when his "victim" jumps up and nails him, after playing possum for a few minutes?
Here's part of the scene that inspired this: http://www.youtube.com/watch?v=oqwJaTRyThc