This would be very processor intensive, so it would only work for small sims.
Each bot is attracted towards other bots by inverse square law. Basically gravitational pull, determined by each robots' mass.
Obviously this would be a just for fun mode. It doesn't mirror anything in real life (anyway, I sure hope not!). I think it would be fun to see bots learn to use a gravity sling effect from large groupings of vegs to accelerate to high speeds cheaply.