Something ive already brought up is weapon costomisation. But something i came up with just now is that a weapon could have durability and if your weapon breaks in battle you continue the fight with your bare hands and you have to spend an upgrade point to get a new one or repair it. Maybe its a bit unfair for the player but it would be cool to punch and kick your way through the waves.
Also something else to add is a kicking animation and being able to do damage with kicks.