Here's the basic chassis for the 15x15x15 in robot the PC robotics club is working on for the Toss Up College Game. It's a bit different than the High School version because the robot has to be smaller! Anyhow, the mechanum drive is on the rear for scrubbing, while the omni wheels pushed out front should help the robot balance as it's sucking up the buckyballs and big spheres and then elevating them during the competition. Should work, should work, should work...!!! Not bad for a few hours of work, eh?
Yeah, the scrubbing still needs to be programmed in by the programming team, but hey, they did this after just a few hours of build!