Kv depends on timing (sensor timing or esc timing) and the physical characteristics of the motor construction. So the best way to standardize would be to label the motor by the number and type of winding as that is a physical attribute specific to each motor. That, in a sense, is what we already have as kv is just an approximation of what the motor can do.

There are so many different motors it would be almost impossible to standardize more than we already are. If you have a sensored motor it will have certain characteristics different from a 12 pole sensorless outrunner.