>>274862
Fate is extremely well prepared for an MMO format. It'd be a map based MMO rather than a fully open world the main hub is Chaldea, and from Chaldea you can rayshift into various singularity missions. Some are minute and contain only the one task, others are major and have a town of their own to act as another hub from which you operate.
The one difference though is that it would operate on the prilya system, or a derivative of it hybridized with the Demi servant system, were you a human borrow the powers of heroic spirits. In this timeline, because every fate entry takes place in a separate timeline to account for inconsistencies, the Demi servant system has been perfected, and every player is a demi-servant who can channel heroic into themself at will, borrowing a portion of their power. As the story progressed, you would improve your own Spirit origin such that you could handle multiple heroic spirits at once, each giving a different aspect of their power in the form of skills, equipment, or super moves unique to that character.
Once you are strong enough you can dedicate all 5 Spirit origin slots to 1 hero to become an avatar of that hero, letting you really feel like you're ~batman~ Artoria, a powerful but not very diverse skill set. Think of it like multiclassing. Will you put all 5 skill points in Artoria or will you do 3 Artoria 2 1st Hassan?
Numbers notwithstanding, there really are a million ways you can implement the servant system into an MMO that would be satisfying for fans to engage with.
Imagine finally making the rin-gil-redman golem from the wiki's popular pages tab as your character. I'd play that game till EOS and ruin my life over it.