Steel Bolt Crossbow Turrets- Always wanted this as I never want to waste ammo on auto turrets. This wouldn't make the other turrets obsolete as the fire rate is much lower than other turrets since it reloads after every shot, so it would have lower dps.
Recycler Laser Rifle- If You play fallout you know what this is. I think it should deal the same damage as a pistol but recharge ammo over time to a maximum of twenty or so. It's the type of gun you use sparingly as the ammo would take a while to regenerate. (much longer than in fallout) so it wouldn't make the pistol obsolete as the pistol can be fired off a lot as long as you have ammo. It would use a solar cell in the construction of it and have little solar panels on the top of the gun.
Flamethrower- Would be better than the flame bottle but would consume a lot of fuel. Won't accidently set yourself on fire. Continuous fire will add additional fire DoT to enemies.
m60- I know this is slated for a18 but figured I'd list how I want it to work. Should slow you down a a lot. The movement penalty should not be 100% negated by perks as it should be the type of gun you don't run around kitting with.