It would be cool if weapons had levels to them (since we don't have character levels yet) and at certain intervals you would unlock what would basically be a small upgrade for you preferred weapon. It could function as a separate card slot from the other two but these would be weapon specific (of course some of the cards that apply to character weapons would be shuffled around and this would clean up the card selection for characters and give more character variety). There could be only by 3 or 4 per weapon maybe even offer alt-fire modes. You could only equip one though. For example there could be a card that makes Mikah's first weapon fire in 3 round burst(on a toggle) or another that increases the bullet capacity by 5. These would also slightly change the weapon model. If it where something like an increased mag size the weapon magazine would be a little bigger. Stuff like that.
You could also have cosmetics for weapons.
I'm sorry if it seems a little rambly. I'm just spit balling here guys seeing what sticks.