one piece can simply be something gold roger said so that pirates will set out on their journey to grandline to try to achieve it. and in the course of doing so, they realise their dreams, and fulfil their dreams. wouldn't this be something all of them will want to achieve rather than get their hands on a treasure that they don't even know about? oda has always stressed that they are out to sail because they want to fulfil their dreams, and it will be the best reward for them rather than getting any tangible treasure. they may realise in the end that one piece simply means their dreams are fulfilled.