I mention this the other day. It all stated with the promise of Joy Boy, he told Poseidon (The mermaid princes of 800 years ago) that an descendant of him one day would fulfill his promise which take Gyojin island to the superficie with the Noa.
Then we have Roger that found the One Piece, and as Rayleigh said, they were not ready for it, they were ahead of the time. Rayleigh sounded like they made a discovery that would change the world but there were some requirements and those were Momoshiki and Shirahosi.
Now, one might think it is gonna be Luffy, but is this true? If Luffy is a descendant of Joy Boy, then Dragon, father of Luffy would also be a descendant of him and he fits more in this prophecy than Luffy since Luffy goal is to have an adventure with no ties and if he has a tie to destiny, then it wont be an adventure since this would be his destiny.
Dragon is the man who is fighting the system and the one who is gonna change it, while Luffy is gonna be the pirate king.