It wouldent be major, all epic and legendary weapons that dont do passive effects already examples being stasis beam, voltaic rpg, gravity beam ect, will have 1 slot, it can be filled with a weapon mod, the mods will do things like stasis effect, caustic effect "a slow burn",  aiming debuff "like what voltage weapons do when they hit". Or ability disabler which disables a mecs ability like "panthers stasis fence" putting it on cooldown. Needless to say the weapon mods once used go on cooldown for 25 seconds. Or whatever you think would make them balanced. Weapon mods should be rare and hard to obtain and cost lots of A coins mabie through crates or events if it's crates in store def 3k A coins for a 5% chance to get a random weapon mod.

My reasoning behind wanting some kind of weapon modification is it feels like your weapons are... white bread, they are boring and need some kind of change. Shoot even weapon skins would make them feel less like.. im watching paint dry.