Rather than wheels, what about clever uses of vibration motors?
Four legs, each with the base position just going below the chassis. Spring load them, and add the motors. By adjusting the vibration frequency and (the vibration equivalent of position, um, the.. dammit) of the motors in relation to each other, the bot, I think, would move in a different direction. It’d have to be on a fairly flat surface. I wonder how it would work with no openings, just vibrations though the chassis itself.