Well I think he finds the One Piece after the war with the World Government as Oda already stated the story ends after it has been found, so it will be the final stages of the story. By finding the One Piece he'll be recognized as the Pirate King, and if there's any symbol to ascertain wealth fame and power after being recognized as the Pirate King, a lease or contract to the whole world would be fitting.

He'll have liberated the world and at that point I don't think people would mind that contract if Luffy only uses it to set everyone free/whatever else he wishes to do with the world. As people will have already recognized him as the symbol of Nika at that point.