Gearbox (Motor Multiplexer)
This video above shows how twelve different axles can be driven by one central axle, but it uses two motors, one to drive the central axle and one to drive the mechanism that makes the intermediate cog move into the correct position such as to drive each of the outer axles. So the ultimate challenge would be to make a gearbox which uses a single motor to both select the desired output axle and actually drive that axle. I think this could be achieved with a kind of ratchet mechanism whereby when the motor turns in one direction it causes the different gears to be engaged and when it turns in the other direction it powers the selected output axle. This would mean that the selected output axle can only be driven in one direction, which would be a limitation compared to the gearbox in the video. Check back later to see if I was able to do it!