Guardians should become elder guardians once they have killed enough squids

This would solve two issues: First of all monuments are useless once you have looted them, and you have no incentive to go there again unless you build your base there. Second of all sponges are currently not renewable, which is a big issue for servers who do not want to explode their world file size by exploring.
The amount of squids that a guardian needs to kill to become elder needs to be quite high so that they are still rare, and only appear if you are around the monument for a long time.