Like hullmods but for a single weapon, where after you place the weapon you get a list of mods that can be added to it. They would probably be cheap and maybe there would need to be a limit on how many you can add to a single weapon.
some examples could be adding some mods to your engine facing pd weapons to increase their damage to missiles, or maybe increasing their target leading accuracy. Adding some range to one of your main guns in exchange for turning speed on a turret, or maybe setting a turret mount to only face a certain direction but boosting the stats of the weapon placed on it.
This might just add too much complexity and clutter to the game but I just that it would be a neat idea.