I think that Roger was paid to by the marines to falsificate his illness and death and tell everyone about onepiece, but really it doesnt exist. It was just a marine trick do destroy all the pirates - everyone will go to laugh tale, and only the strongest pirate will survive. And then at the last island there will be some superstrong marine (or maybe Imu/Seadevil himself) to kill or imprison the strongest pirate. And I think that at laugh tale Luffy will be imprisoned in impeldown again.