The next logical step would be integrate the motor directly into the wheel. With 4 wheel drive motors it wouldn't require huge motors, they could each be small enough to be packed right up against the inside of the wheel.
I'm actually surprised they would even need the gearbox. Why not just put four Outrunner[1] motors, one on each wheel, and let the wiring be the only thing that has to flex when the wheel is steered.