Urk. MMO, even one like Warframe would be a horrible choices for ME. A character driven game where you're interactions with companions is supposed to have an impact at the end of the story wouldn't work at all for a multiplayer game. You'd just be transposing the look of ME and pretty much nothing else.
You don't make it a campaign driven story, you make it a world driven game.
Think more EVE.
Take the ME universe post Reapers, people make characters, and then they get to go out into the chaos of the Post Reaper galaxy (relays are fucked, capital worlds are burned, governments have been gutted, Shepard is dead, etc).
Like, want to go rebuild a relay and charge tolls? No problem, get together a guild, gather enough resources/infrastructure/skills, and rebuild that relay. Then you can either sell access (via toll) or perhaps keep it hidden that you have rebuilt it so as to help your attempt at conquest. Except you also need the forces to defend it, because other guilds can attack you and try to capture it. Maybe NPC entities in the form of the governments also try if they see an opportunity.
You don't try and tell a
story as such, you let the players make up the story in the ME setting.
