Hmmm...tennis balls......I think it would be cool to have tennis balls that would need to be scored in goals that are along the perimeter of the field (that can be tipped over or rotated to spill the balls)...Then it would be cool to have the player stations above the field....so u have to look down to control the robots....(maybe have a glass floor so u can see through it)....
I know it seems way out there, but i think it could be a possibility.
(Edit: Editted to stay on topic.)