I think the game needs a pipe shotgun. Two pipes and a hacksaw and maybe a nail. I’d second a muzzle loader as well and a way to craft one.
Futuristic firearms should just replace the roles the odd ball antique guns have in the game, like the .22lr submachine. Any energy weapons or really sci-fi type gun should be interesting to use, not just a pew-pew lazer version of a regular gun. They should work like a ranged jackhammer that blows apart walls, like the eye enemy does, or the shots instantly set the targeted square/ target on fire. A microwave energy cannon that boils crowds of zombies. Something.
And to balance them out, good firearms should be more scarce. Futuristic gun control, so gun stores and zombies, houses have mostly low end shotguns, hunting rifles and revolvers and not so much powerful assault rifles and full-auto submachine guns everywhere. Makes the game too easy with all the exotic high end weapons all over the place.