I think I posted this in the wrong place previously. I was on Mangahelpers.com and I came across an interesting theory that somebody made about what One Piece is. Credit goes to Freid on MH.
One Piece is the Ancient Kingdom.
One Piece is not a treasure, it does not have anything to do with Roger’s most valuable possession, be it his strawhat or a Portgas D. Rouge picture, and neither is it meant to be something materialistically gratifying for the person that finds it. One Piece, the will of ‘D’ and the distant Great War are all interlinked.
Whitebeard says on this page that Teach is not the one Roger is ‘waiting’ for. This implies that the nature of One Piece extends beyond it supposedly being a reward of some kind. There is a task that needs fulfilling which one particular person is destined for. This person would find One Piece, otherwise known as ‘The Ancient Kingdom’. The discovery would be the catalyst for the greatest war ever to be seen.
Whitebeard’s final words foreshadowed what One Piece is. They were: ‘Sengoku, you people of the world government are living in fear of that great battle that will someday engulf the entire world. Though it’s nothing to do with me, when somebody finally finds that treasure, the world will be turned upside down. Oh yes, it will be found. That day will come without a doubt’.
The discovery of One Piece, as implied by these words, is going to be the trigger for the future catastrophic war that will engulf the world, which the World Government are afraid of. During Whitebeard’s final proclamation, Sengoku gets agitated by Whitebeard insisting that One Piece will definitely be found. This suggests that the World Government are fully aware of what One Piece is, and its discovery is significant also to them. If One Piece were the Ancient Kingdom itself, then its finding would be highly consequential and meaningful to them, considering the fact that we have been explicitly shown that any potential knowledge of it warrants death.
Moreover, here, just before Professor Clover was able to mention the name of the Ancient Kingdom, he was shot dead. Why did Oda intentionally keep the name a mystery to us? What other name can the Ancient Kingdom possibly be called, for Oda to expressly decide to keep it anonymous?
The real purpose of Roger initiating the pirate age was in certainty that amongst the many that set sail, would emerge a particular person that inherited his will and would find One Piece; The Ancient Kingdom. With the discovery of the Ancient Kingdom, would the truth be unveiled, in which the war which Whitebeard had predicted will take place upon the uncovering of One Piece, will ensue. ‘A man will come forth to challenge the world’ and The Government will once again attempt to eradicate what is left of the Ancient Kingdom but history will be rewritten.