
Idea to add more depth to weapons: Improve skills using XP.
The system works as follows: the XP you gain from killing monsters with that weapon accumulates. You can then use it to upgrade the weapon's abilities, requiring more XP depending on the level.
As you improve your skills, they will have new interactions, that improve with each level. For example:
Invisible ink lv1: While you are in a state of invisibility, one out of every three attacks will deal critical damage.
This would make XP, a resource that currently only serves to level up in passes, also important for progressing in the weapons you use regularly.