Two of the main limiting factors as of now for geared electrical motors are the torque output and high rpm. The higher the torque output, the stronger the gears/shafts/bearings need to be which in terms adds roational mass that can not spin at infinite rpm.
The general concept of dc (brushless) electrical engines have current output limit that in term limits the 0 to low rpm torque output. As rpm's rise, torque increases as to a point that voltage becomes the bottleneck. Top end rpm is achieved by dropping the current, as so torque drops dramatically.
Ive been toying with the idea of a CVT gearbox, purely for simplicity sake, engine torque can be limited at all times so it will not cause slip in the belt driven conical gears of a CVT, top end rpm can be given a little boost in torque as CVT can provide a more suiting gearing ratio... its all just theory.
Rimac use a nice and well packed design on two gears (low and tall) gearbox, that seems to be working well for them and their customers.