Of the accessories that might add to the utility of the KSG would be some sort of internal flashlight activated by a built-in switch.
That might be best done inside a new forearm, or by some sort of "flat" light that could be mounted on the top rail.
I find that external lights mounted on the bottom rail take up too much room needed for gripping the gun.
Most shotgun lights tend to be not much more then slightly modified standard flashlights.
There's nothing that says a light has to be a round tube.
Another idea I had would be a spring-loaded butt pad to reduce felt recoil.
The 870 and Mossberg conversion kit sold by Bullpup Armory has such a pad.
In the case of the KSG it would be somewhat easier because of the easily removable stock pad.
The pad assembly would need to work around the lifter and would need to not be much longer/thicker then the Kel-Tec extended accessory pad.
An easily made accessory would be a hand loop for the forearm to act as both a pump assist handle and a guard to prevent the hand from slipping forward or back.
These have been made for other guns, and you can rather easily make one from nylon strap, but a molded plastic loop as small as possible but still large enough to allow easily slipping the hand in and out and direct mounting to the lower rail might be a good seller.
You're very right that most KSG accessories are for Range Toys and have no use in the Real World.
I can see sitting in front of a judge and jury explaining all about how you stabbed someone with the spikes on your KSG and them looking at you like your wearing a Hitler costume.
Again, an AR-15 type lever safety would be nice, if not at least a standard safety that could be either reversed or one made specifically for a left handed shooter.
The idea would be to enhance the KSG, not make it a better toy.