First they should add a Pulse cannon. It would be similar to a missile turret in design but fire pulses of 3 tracer slugs every 1/2 second. Make it more powerful than a Gatling gun in damage but not as powerful as a rocket launcher. Make it 3x3x3 like other weapons.
Second they should some sort of flak cannon similar to a ww2 bofers. These would shoot explosive slugs/missiles that would be shot like a Gatling gun but much much slower rate of fire. The slugs would explode at a maximum range set a terminal. The explosions would damage all blocks similar to a missile but also be able to destroy incoming missiles if they were close enough. The slugs would also explode if it hit the target. These weapons would fire like traditional bofers.
Third I think their should be some sort of lazer beam weapon. This weapon would be a sniper weapon that would fire very slowly and require a recharge like a jump drive. This weapons power could be changed to be fired faster but less powerful. This weapon could only be mounted on large ship. Something like a MAC cannon from Halo. This block would be 1 block wide 1 block tall but 10-20 blocks long.
Fourth I think their should be more broadside weapons for large ship to ship combat. These weapons would be 2-3 times more powerful than a missile turret but fire slightly slower. These weapons would be 2 blocks wide 2 blocks tall and 1 block thick.
I do understand that you could download mods to get all of these weapons, but when your joining another persons world and they don't have the mod/mods the weapons will not appear. I understand that adding these is hard but it would be nice to have a variety of weapons to customize our ships. Please up vote this so the developers will maybe add new guns to the game.