My latest theory & speculation on One Piece:
I think Luffy's goal of finding One Piece might be closely related to Robin's goal of finding the true history. -Maybe- even One Piece itself is the last Poneglyph Robin's looking for, the one and last piece of Poneglyph.
Now, lets talk about Gold Roger for a while:
Gold Roger was the most powerful pirate, yet he got captured by the marines. Considering how powerful he was, it's hard to believe that he was captured at all. I'm guessing he might surrender himself or willingly get captured. Maybe the world government threatened him through a hostage or other similar situations?
The Reason of Capture
Gold Roger was able to read Poneglyphs, and he had travelled to the end of Grandline. Therefore, we can conclude that he knew about the true history, and that's the main reason why the world government wanted him dead.
The Quest for One Piece
Right before his death, Gold Roger announced the pressence of One Piece and thus started The Pirate Age. I'm guessing he did this as his last effort to pass on the true history. He hoped that someday someone will be able to reach and obtain the One Piece, and maybe educate the world about the true history.
Back to present timeline…
So far the only facts we know about the One Piece are that it's the greatest treasure, and it's located at the end of Grandline. Since the Skypiea arc, we learned that the last Poneglyph is also located at the end of Grandline, in the island of Raftel.
When I started reading One Piece, I was guessing that the One Piece is more of something abstract, not something material. It could still be something like that, but lately I've can't help thinking more and more that what One Piece is is very closely related to the true history.
Still too many missing pieces of the puzzle yet to be revealed.... One can only guess.
UPDATE1: The last phoneglyph being the One Piece is really a small possibility, imo. I don't think Oda-sensei will reveal One Piece this early, and I bet it's going to be something surprising and not easy to guess. My conclusion (my guess): One Piece is not the Rio Poneglyph, but it should be something closely related to it. I'm guessing by the time the true history is revealed, we'll be able to guess what One Piece is.