So, Its not a Marco Ace prompt though I really want to see this complete. Its going to be a mystical/mythological AU. In which Luffy is born in a modern world that also has gangs that are made up of mythological monsters and the human world doesn't know about those gangs that reside inside there world. Only a secret organisation does. Still not intrested, well how about the gang leaders always meet once a month in a secret cafe, called the going merry where Luffy a human waiter that works there.
I'll let you decide which carectors go where and what the plot is. Inform me when you decide to take up the prompt, this may turn out to be a Marco Ace. I may take up the idea myself. Good luck!