Make a way to upgrade weapons, like paying with an item and some money to increase a stat on it. Lets say you take a sword and some sheet metal, and pay 50 bucks, and it'll deal more damage. take a rifle, add snake oil, pay 50 bucks, and it will reload faster. and then to make it fair, have enemies deal more damage and walk faster slightly with each checkpoint. This will add a lot of progression and excitement that i feel is one of the games weak points currently.