JoyBoy is a man who lives hundreds of years in the past The only feasible chance here is descendancy No other option can be possible in my opinion. Unless Luffy as a new born was sent into the future Joyboy was reborn as Luffy, Luffy's special power to turn enemies and allies into friends is Joyboy's special power. That power is also what is used to control Shirahoshi( Poseidon). Luffy awakenes her power fully.
