This would be pretty bit thing to develop. A bit unrealistic idea but I’ll throw it here anyways.
You find or make a new and great piece of equipment.
Then you use it and the zombies tear on you and your equipment. The equipment is damaged.
You spend few minutes repairing it and the equipment is good as new. Maybe even better!
I think this devalues salvage and turns the player into one person industrial powerhouse. Once you have the equipment and materials for maintainance you are able to sustain your current level indefinitely.
The repair system is also very simplistic. Item has material, repair requirement is solely based on that one material.
Metal item? Repair with welder/soldering iron.
Engines have faults. You can’t just repair them with tools. Instead you need to get your hands on some spare parts and then fix the thing.
Lets expand this to all items and add new details to the faults system.
Damage on items can not be repaired at all. The damage level becomes a more abstract “how worn out this thing is” indicator. Once item is damaged enough it is destroyed. Damage level has no impact on item usability.
All items have faults. After item takes certain amount of damage it receives a fault. Each fault has its own repair requirements. When a fault is repaired part of the damage that caused the fault is repaired. Higher skill repairs larger portion. Each fault has its own effects on the item.
Jacket worn by the player. After taking 10 damage on the item it receives “tear” fault. This reduces the effectiveness of the equipment and “10” damage is stored in the fault.
This fault is easy to repair and with just a sewing tool and some thread the jacket is good again. But only 8 of the 10 damage is repaired in the process. The remaining 2 damage will stay on the jacket permanently.
Same item could have multiple faults at once.
This would mean that even after you get good equipment you still need to occasionally salvage areas to get new replacement items. you won’t be ble to just repair the same item with the vehicle welder indefinitely.
Tear/hole/large tear (reduced warmth and protection), missing sleeve (removes bonuses from that body part), torn pocket (reduced capacity).
Some clothes could just fall off randomly if they have too many faults (gloves, helmets, shoes, strapped things).
Faults on tools could cause extra battery power to be consumed and items that are turned on could turn off randomly or require multiple attempts to turn on.
Wheels could get punctured causing them to have extra drag and wobbly ride.
Cracked armor could let some hits through without damage reduction.
Tanks and liquid containers could leak or have reduced max capacity.
Some faults could add to the item name (for example “torn jacket”. Max one fault addin to name) but mostly they would be shown as “-” symbol at the end just like mods add “+” symbol.