drum mag on the side perhaps? i think it could be done well if you dont just slap one thing on and think it works on it own.
id love to do a tactical tommy properly, not rails but modern pistol styled grip, red dot sight built to fit to where the rear metal sight is, a stock thats shorter and maby more like modern sniper ones(slightly collapsable) and under barrel either rail for grip or something like an aug grip. metal work other than that is untouched... only other thing is probably to much hassle, to bring the fire slector triggers lower so they are usable by the trigger hand...