Could you add a tandem coilgun - a gun that uses normal bullets and gunpowder for "first stage" acceleration and additionally accelerates bullets with gauss coils. In game terms it should be like x3-x5 damage, 0.5-0.3 fire rate, +20-40% firing range compared to SMG, "heavier" shooting sound, and use lots of copper wire (for coils), portable fusion reactor (for power source), some processing modules (computing coil cycles) and some steel/iron as materials in recipe. Kind of the opposite of minigun - great for fighting behemoths and enemy turrets with lots of armor, not so against small critters, still can use a wide variety of standard bullets and do not run out of them as quickly as fully upgraded SMG.

That seems very cool actually! Already starting to work on it.

Some other ideas:

Heavy flamethrower: basically flame turret gun: long range, arc-firing, splash, massive damage. As a trade-of - massive movement speed penalty while firing it.

Grenade launcher: craft launchable "ammo"-type grenades out of regular grenades, claster grenades, land mines, poison capsules and slowdown capsules to fire them from specialized launcher at higher range. Could be tricky to code, though.

Nanite projector: a gun that fires clouds of nanites that quickly repair your stuff in AOE, for those who like to turret creep. Should be quite high-tech.

You have some really cool ideas! The grenade launcher is possible to make but I can sadly only make it fire normal grenades with my current knowledge, but the other ideas are possible. The nanites might be kinda hard to code but I think it is possible. BTW I finished the Tandem Coilgun.