are you sure real world examples are legitimate when we're talking about fantasy or science fiction games?
I think verisimilitude is doubly important in fantasy settings, as without it they'd be too alien to be relatable. If the rules are to be fundamentally different from reality, great care has to be taken to depict this. Otherwise, rational assumptions will lead to problematic inconsistencies. Worse, the reader/player could be discouraged from making any assumptions, causing what should be the logical consequence of cause to become a string of confusing non-sequiturs.
but where are your examples of existing MMOs?
That depends specifically on the point of contention.
Shadowbane had a lot of player agency, and while it was horribly balanced, it provided much more in the way of opportunity to influence the setting and other player characters than most games do. It did this by not really having any overarching plot, and not allowing interactions between characters to be opted out of, except by not playing; there were no instances, modes, and very few safe areas. It also had equipment that was conditionally unique--nothing was ever intended to be unique or particularly unobtainable, but due to snowballing imbalances this is how it came to be.
On the server I played on, when I played, there was a single player faction that had secured a monopoly on many aspects of the game and rigorously enforced their supremacy...they were nearly unstoppable and would destroy other factions long before they could become serious rivals. When the group I was part of was inevitably laid siege, we had no means to counter it because no other group had been allowed to advance to the point they could produce the tools necessary to do so. However, we were able to ambush, kill, and loot enough of our enemies to steal enough siege weapons from them to repel their initial attempts at snuffing us out. I think we had three 'siege hammers' (personal weapons for attacking fortifications in this game), which no one outside the dominant group on this server had ever even seen before...sure felt like wondrous artifacts of immense power to us, even if our enemies could buy them a dime a dozen.
It was a blast playing the underdog, especially when we won. I recall a field battle were we routed a larger force of significantly higher level PCs because half of our opponents we're too afraid to damage their weapons and armor, while we fought mostly naked and thus had nothing to lose.
Jumpgate, despite being of a totally different genre, was similar in many ways, though it traded geopolitical agency for broader economic and social agency. The official factions were fixed, but all mainstream equipment had to be produced by players, from extraction, to refinement, manufacturing, transport, and sale to a destination market. It was also a small, tightly knit, community that had a higher proportion of more consistent roleplayers than any video game I've ever been a part of. Anyway, there weren't any NPCs, except the Conflux (the setting's rough Thargoid equivalent), so nearly everything was player driven. It was possible to have highly customized vessels of functionally unique capabilities through the use of rare 'pre-collapse artifacts'. Anyone could potentially find and use these, but they were difficult to acquire and even more difficult to retain. Insurance in this game didn't replace equipment, it offered compensation for it, and like Shadowbane, there was no way to actually play while guaranteeing one's safety. Couldn't forcibly take them from a ship though...even if you could occasionally coerce or intimidate someone into giving one up (which would more often result in a war between factions).
Space Engineers could also be another example. I haven't played much of it, but I know people use persistent servers to essentially turn it into a small MMO, and it seems like a good example of a game with constraints that make it self-balancing.
