I think someone who needs all the functionlity would simply buy a motor contorller with the features they need in it and wire it to the arduino, rather than use a shield. The only shiled I haveever used is RAMPS honestly, I use seperate boards otherwise, since they just lack the flexibility or adapatbility. and there are a million motor controllers out there. Now say you made a feature rich application specific shield, you may do better. Say 2 or 4 motor controllers w/ accel/decel control, regen, and some accellerometers and gyroscopes, but I can't tell you how big the market would be.
But I bet your documentation would be top notch!
my .02