Tennis.
Here's the idea - Mine-imator tennis. Somewhat akin to the gmod tennis videos on YouTube, those who have viewed them will know what I'm talking about.
Basic Outline: One or more people post their skin on a Tennis topic. The first person to post their skin makes the first serve - once at least one other skin has been posted on the topic, the server creates an animation using both skins. For example, a fight animation.
After the animation itself has been posted, it is the opponent's turn. The server will send the opponent a link with the last frame of their animation so that the opponent can build off of it. This would go back and forth for.... I don't know, 5 rounds?
Anyone up for tennis?
http://imgur.com/URLAZlN