Why not a Rem 870 NM with sear lock safety and a series 80 type firing pin safety? It's basically just a redesign of the ignition and bolt....I'm not positive, but I think there's room.
If properly marketed, hunters and police would flock to it.
Unlike most here, people who use their guns hard, fall on them and drop them out of the truck. I know I would like this safety change, but for an 18.5" rifled semi-auto 12 ga....the safety should be a thumb lever or possibly a trigger finger push button.
Just market it with hunter's running, dropping to the ground, climbing through muck, "pushing" heavy woods, shotgun falling out of a tree....is your shotgun tough enough for this?