In order to be able to weild the Rinnegan one must have the power of the Uchiha and the Uzumaki clans. Obviously Naruto has the Uzumaki clan power, but where did he get the Uchiha power? Itachi. That mustve been what Itachi betowed upon him. In order to unlock the Rinnegan a truly tragic incesent must happen. So something will happen to Sakura, or Kakashi or someone close to Naruto. Then he will come across the ultimate power of the Rinnegan