Peter, I would love to see that book!
I had to do some heavy thinking myself before all of these cool gear generating programs came along. You have to work with pi, diameters and circumferences, all kinds of crazy mathematical equations that can cause a man a headache. Been there, done that.
You're right. It's easy enough now to generate the gears with modern software/programs. How creative and imaginative you can get with the gears' center designs, themes for great looking and fun displays, as well as coming up with the right hardware to put it all together and make the displays run smoothly are the challenges left.
This has been an amazing thread that has addressed most if not all of the challenges for hardware. The rest is left up to your imagination!