Yin and Yang
This is an idea for a pair of matched robots, "Yin" and "Yang". This pair will be almost identical in phyical construction, but very different in programming.
Yang will be aggressive, and constantly seek to attack Yin. Yin will be passive, but not defenseless, preferring to dodge and avoid rather than directly confront.
The bots would each have some sort of emitter signal, probably IR, so they can track each other. They would also each have a prominent button, which the other bot could hit to deactivate it's opposite temporarily.
Yin (the more passive) would have the button on the front, requiring Yang to approach him directly. Yang (the agressive bot) would have the button on the back or sides, so that Yin might strike the button as he dodges a charge from Yang.
This is just a concept right now. I'll probably use my SHR bot to initially test some programming. If this idea moves into new constuction, I'll create a robot page for it.