Back to the subject- a governor for the rotor, as opposed to the engine.
The real problem is that there are times when you will want to over-ride such a governor- say in an extended range autorotation when you want to stretch the glide and use the minimum NR. If the rotor governor were set to keep the minimum NR, then you'd be fighting it during this maneuver.
What happens when you want to use the rotor speed to cushion the touchdown - how do you really over-ride the governor then? How much force would be needed (and that is really the only way I could ever see anyone certifying something like this - having a switch to disconnect the rotor governor at the very critical time in an autorotative landing is not suitable).
And so on.
Sensors would need to be reliable, logic simple, and it would have to be very easy to over-ride.
Worth thinking about, perhaps.
But what about then going to fly a machine that didn't have one???