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.
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Yes, this is something I've been wanting for a really long time, I've been playing around with different magic system implementations especially because of playing Arx Fatalis, trying to get a dynamic magic system that feels natural, it's just really hard to get right and from experience, gesture based systems might seem fun, but they fall apart under certain circumstances and are limited to specific actions, so I've actually been considering different types of input systems and effects, for example graph based systems with multiple layers for construction and then for execution using key combos or hotkeys to combine sub graphs or just execute a single graph to perform actions or initialize causality based systems.
I actually thought about something similar a while back, specifically with a magic system where one player creates puzzles to prevent griefing, sort of like wards or charms in fantasy, and attackers have to solve them to be able to get to the loot and get damaged by the elements if they fail, in a type of Rust PvP setup. It ended up way more difficult when I tried to make a prototype, mostly because you have to allow for a wide range of possibilities while also allowing a wide range of difficulty, but this is something I think would be fun to play, it might just be really hard to implement and balance.
I'm not sure if you're bringing this up because of the new Sims 4 expansion, but I thought Sims 3 Pets did a pretty good job with the horses and comes close to what you're describing, but I'm guessing you want something more in line with a traditional RPG.
Not sure if It's what you're looking for, but have you tried Stormworks: Build and Rescue?
Yes similar, but not entirely. SimCity is more macro-economics, I was thinking more about micro-economics, like the demand and supply within the city, similar to Factorio where one factory needs resources from another to work, but in this case prices would fluctuate depending on the supply and demand of certain resources within the city, then you could use these differences to make money for your sims, or create a business to fill an unsaturated market, for example producing sweaters in the winter to fill the clothing demand while having to repay a loan you took to begin the business without going bankrupt. This has always been what I wished I could do in the sims, Sims 3 came close with shop ownership, but it didn't really simulate the accompanying economies.
So something like Stardew Valley (I haven't played Animal Crossing) in the middle of the Amazon rainforest, Australia or Island where you have to survive the harsh conditions but build up your house, maybe even build a village, where you can then start an economy and make the best of the conditions given? Maybe even with some natural disasters and extreme weather destroying everything if you aren't prepared like Banished. I think this could actually be fun, maybe even add some multiplayer.
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.