There is another post mentioning weapon customization and I think this should be a part of that. The game already has a stand alone single shot grenade launcher, I think the ability to mount them on certain guns would add a cool layer of survival fun. You have an m4 rifle and your running along and a plage jug shows up, as you open fire on it, a feral comes to investigate. You didn't bring enough ammo for this! You manage to drop the jug but your weapon is dry. Melee weapon is broken and no CQB on this character. Ah ha! You have a 4 shot 12g shotgun mounted on your rifle! Boom boom boom boom! Feral dead and you live to fight another day.