I don’t know if you’ve played Rust or ARK survival, but those systems fall apart pretty easily and leaves a lot of players unable to enjoy the game due to the group systems being exploited in some way or another. In a lot of cases especially because you have to join a guild for protection if you want to progress, and that leads to more realistic situations where players are exploited and end up leaving the game, because they don’t want to experience reality in a game, they want to escape it. It takes away form the PvP experience by turning it more into a clan fighting game like Last Oasis which was also a mess due to the clans basically destroying all the smaller players before they can actually progress, because groups were extremely overpowered. Last Oasis is basically dead at this point and they did have player based protection system, and it was abused. Therefore a system to equalize the playing field would be cool, and in a fantasy game something like a single player or a small group being more powerful than an entire clan would actually make sense.
From what I’ve experienced, having players protect other players is not going to work, most people are just going to find ways around it like in real life or actually make things worse for the people they’re protecting, there is no real integrity if you have enough people and corruption is always present. Your best bet is having save zones enforced by the game mechanics, but what I’m suggesting is giving players a way to protect themselves against people abusing systems without relying on other players, because trust in other people is not something you can rely on to keep things fair in a game, while theoretically allowing player owned cities, while keeping them in line with basic expectations.