What if luffy already has the one piece. Think of the iconic straw hat as a crown of some sorts. Gol D. Roger said that he left everything in one place. What if that everything is the one piece and he left it on the head of the king of the pirates assuming Luffy becomes king of the pirates. But then why did Shanks get the strawhat? My answer to that is that Shanks was the one who passes it down to the next king. Roger could have been destined to pass that hat to luffy if he was not executed 20 years before luffy was born.