I think that the weapon modification should be able to really change how to weapon feels not just tiny improvements to the gun. Think a cool idea would be to add a system like call of duty has where you can add a few attachments, then to add onto it you could have one or two upgrades that really change the gun such as, making the gun automatic, shooting multiple bullets, and making a gun do more damage but have a lower fire rate.
Thank you for taking my ideas into consideration.