Single shot (manual fire?) missile launchers that deploy a large minefield where the missile explodes.
Fighters bolted to the hull with one-off explosive bolts, they give a wing of fighters but only once per battle, no replacements for that wing ever, and there's a huge CR penalty when one gets destroyed to represent fabbing it with a not very good for purpose fabricator set and cobbling it together in space.
Targetting array - when it's 'firing' at a target, all other weapons on the ship have increased accuracy / lead better (/ reduced recoil?)
Focussed sensor jammer - affects enemy accuracy, only one ship at a time but that ship misses a lot more.
Mine layer 'missiles', for putting on freighters you expect to run away a lot. Terrible in most combat scenarios, good when running.
Flux absorber - fires a laser at another ship, absorbs flux from it somehow.
Gravity ripple - does useful stuff in campaign layer, pulses push away projectiles / fighters in combat layer.
Wild weasel - minor ECM bonus, no-damage long range PD laser that, if it hits a missile, retargets that missile at the ship mounting it.
There's some ideas. I like the idea of more uses for mount slots on things you wouldn't usually fill, or generally just different types of things that aren't just gun, gun, more gun, big gun, double gun.
