My personal idea for this to happen is that every drone should behave as a giant beach ball, physically. The drone would be in the hypothetical center of the ball, and it can move around freely as long as "its ball" does not hit other balls. When it does encounter other balls, the drone performs a physics simulation of a beach ball, making sure that the center of the beach ball will never hit the center of another ball by taking into account "compression" forces, etc.