I am building Tales of the Secret Wars with the core assumption that the players are members of the Blackcloaks (the literal MIB) because that gives an immediate campaign hook into "why do we care?"
Because it's your job. Closing the demonic portal and covering it all up is how you pay your rent.
Invariably if you make a TTRPG setting about being a thing, someone will ask “oh can I be literally anything else”
Dnd players who want to role play tavern owners, aberrant players who want to play baselines, eclipse phase players who want to play Jovians