Whenever people talk about the treasure of One Piece, be Rayleigh or Whitebeard, they also talk about the void century and the will of D.
There is something that match beetween them:
- Gold Roger discovered both
- If one of them is found the world government will be in great danger
- We saw Roger telling Whitebeard what the will of D was, them Whitebeard talked later as if he knew what One Piece is.
- Both have an hidden island (Raftel and the Kingdom which the name was interrupted), worth to comment that the picture of them looks a lot alike.
The void century is not the treasure the One Piece. We know this because Rayleigh would tell to the Strawhats about them as separated things.
Another connection is beetween the void century and Skypeia:
- Roger was at Skypeia, and wrote a Poneglyph.
- Montblanc Norland, 400 years ago, said that 400 years before that (800 years before the current timeline) the sky island had a "old and boring story".
- Skypeia was involved at the great war
- The moon originated some winged tribes, and an isolated tribe somehow resembles the will of D people
Rio Poneglyph is a part of the treasure.
Always thought that there was an obvious hint that this was the case.
I think it's also interesting that Roger could hear the voice of all things and write that message in the Skypea poneglyph.
Maybe His ability is not as precise, and that would justify the fact that He did not get the whole truth but a part of it or somewhat close but not exactly the true. I don't know If I'm making sense, but bare with me.
When he found the Rio Poneglyph and hide it on Raftel I think it was to make sure that a person strong enough to cross the most dangerous ocean in the world will find and fight for the true that it's revealed on it.
Because of his disease, he chosen to stay silent about the subject, but I bet He said something to his crew and it's the reason why Rayleagh is on the place where most of pirates reunite. And Crocus when most of pirates start, too (In Grand Line, at least)
And maybe it has some sort of connection with the revolutionaries, Roger telling Dragon something important. And then We see Kuma telling something to Rayleagh...I think He was an spy of the revolutionaries on the World Government and sacrificed himself for the main objective of the organization. It explain the reaction of Iva and why He said something to Rayleagh on the Sabaody Archipelago.
For me, everything fits perfectly...The truth is already known, the only missing "piece" (See what I did there?) is someone who carries the Will of D. Obviously, we're talking about Luffy.
I think than the Rio PG (which is the union of all the PG in the world) is the treasure, is the One Piece.
In fact look to the chapter 301.