The Master Key

It would be crafted like a regular key, but the gold ingot would be replaced with diamond ingot (or whatever ingot feels fitting, just not a gold one). To use it, you would shift click on an already claimed door/vault (or just use it like a regular key on unclaimed things), and it would ask the person who claimed the door (i.e. the person who used the key on the door) if they would like to be added to the master lock system. (assuming they're online, if they're not, it would say they aren't online, and to try again when they are.) If they decline, it says "Request denied", and that's that. If they accept, it says "Request accepted", and then the master key could open the doors. This could help people who manage buildings on RP servers especially if they need access to EVERY room, while certain employees could only access the doors they were allowed to. Whatever you name the Master Key would become the name of the lock system, and would show up in the description of any keys that unlock doors within that system. You could also shift click over a non-master key in your inventory to add that key specifically to the lock system instead of all the keys with that name. (Note, if you're the one who claimed the door initially, and you use a master key on it, it will say "Key(s) added to lock system!", and then you could go on with your day.)