I believe the idea of the "One Piece" is going to mislead a lot of people. Maybe with all the conflicts going on in the One Piece world (Pirates, Navy, etc), Luffy will be the one to bring peace & unite all the countries, or to put it simply, leave everything in "One Piece".
Everyone has said that Gol D Roger found the One Piece & was the king of the pirates, but he himself has never said that he had the One Piece. He just told people to find his treasure. When Luffy finds Laugh Tale, he may find valuable things belonging to Gol D Roger but the One Piece itself may be something only Luffy can achieve, which is why Roger chose to tell the world to go look for it. Luffy may be the only one to succeed in that task. Roger never achieved world peace despite his popularity & with all the history with the Sun God and such, he probably knew that someone like Gear 5 Luffy could pull it off.
