This came to mind yesterday, but I'm no cosplayer--I don't know how to make costumes, I feel self-conscious wearing costumes, and I certainly couldn't make myself participate in something like the Masquerade at DragonCon. But for those of you who do, here's an idea you can use for a group costume/skit. You'll need five people who can either sing passably, or who can lip-synch to a good singer that you can find. Some skill at performing elementary dance moves in synch will make it work much better, too.
Everyone will be a Green Lantern--you can just go with the basic costume, or be more elaborate and dress up like your favorite specific GL. Then you go out on-stage and begin singing the Temptations' classic hit, "My Girl". But, and this is the key point, you get a giant inflatable rubber ball, paint it with appropriate colors, have it tossed out right before the chorus, and replace said chorus with "Mogo".
"I guess you'd say, 'What can make me feel this way?' Mogo! Mogo! Mogo! Talking 'bout Mogo..."
Yes, it's a terrible joke. But the right audience would lap it up.