The goal is to build a compact, affordable, and simple eBike motor. Many current models are large, expensive, complicated, and made in China (aside from Hilltopper, though $549 is not exactly cheap). I would like to see a $199 mid drive kit (battery included) which simply slides into the bottom bracket and has one cord from the battery to the motor and at most one more to the throttle.
A 6-pole stator, 4-pole rotor, 3-phase in-runner BLDC motor that uses the spindle as a rotor and takes advantage of the negative space inside the bottom bracket is what I'm currently thinking. Using external bearings, a thin rotor, a stamped and laminated silicon steel stator seems to be the best option. However, the space is so small that it's hard to see how to get enough torque and/or power into it, even with a liquid cooling hack like pouring a collant down into the seatpost tube.
So far, the kind folks at Nova Labs helped me use a lathe to bore and trim an aluminum cylinder, and then a CNC mill to drill it into the shape of a stator. Even with ferromagnetic material, it's looking like some type of gearbox or hangdown below the bottom bracket shell will be required.