I am going to strafe from the whole gungineer idea for a second. What if they made a boarding class, whose goal was to disrupt enemies through direct combat. You could make another slot around that too. If everyone was given their standard loads, with one "board weapon", they could fend off enemies, or if their ship was almost certainly going down, make a last ditch effort to board the enemy ship and eliminate personnel. I've played a lot of video games that in their infancy were slower paced, and to solve this became COD lookalikes (never forget the old ace-of-spades). Although it would mean that some people would play like the gun-ho shoot everything on screen, but would be punished unless they can actually fit their niche. If this boarder class existed, he would only have one repair tool, one ammo type, and one pilot weapon, making him pretty nerfed unless ships got near each other.
Cool Ideas for boarding slot weapons
Blunderbuss
-spread of pellets allows for effective close combat
-Slow to reload, slow to switch weapons (instead of instantly being able to fire weapons, they need to be equipped prior to firing).
-Good damage per pellet
-One shot per reload
Steampunk Machine Gun
-10-20 shots before reloading
-fairly quick fire rate
-low damage
-moderate weapon switch speed
-inaccurate
Vintage Rifle
-Long distance
-able to shoot from your ship to theirs
-high damage
-slow reload
-one shot per load
-slow to arm/switch weapons/deploy
-scoped zoom
-accurate
-fast projectile speed
Captain's Revolver
-6 rounds
-Slow reload
-Very quick deployment (allows people to jump off of guns/wheel and shoot within seconds)
-Good fire rate
Cutlass
-Melee distance
-Quick speed
-High Damage
Hammer
-Melee distance
-Slower speed
-lower damage
-decent rebuild
-poor repair
Boarding could become something that attracts the adrenaline gamer, but I think it would be kinda fun to board enemy ships and have crew fights. Perhaps they could introduce individual death timers, so that if you fall off of your ship, or are killed by small arms fire, you can come back after a short time. I believe if ship ramming is a viable option in GoI, boarding party tactics should also be incorporated.
Maybe they could even have a ship with a hatch at the bottom, but lack high damaging weapons and armor. It could be a dreaded pirate ship.