One piece is the definition of "It's the journey, not the destination." Nothing the One Piece could actually be would be worth 23 years worth of content hyping it up.
(I myself am still a fan of the "one-(head)-piece" that make someone a king, but has to be an item that has been around since the first season theory. Gee, I wonder what that head-piece could be?)
Yea luffy's hat. It's been a while since I read the Manga, I could have sworn gold d Roger had the straw hat when he got executed but it wasn't present in the live action show.
Gol D Roger had the straw hat way before visiting Laugh tale. That would be worse than "It was all a dream", or "The one piece was the friends you made along the way". Both of which have been debunked by Oda.
Personally I subscribe to the theory that the World Government is whom split the world into 4 pieces during the Void Century. Whatever the One Piece is - it will have the power to rejoin the 4 oceans.
It would solve every dream of all the Straw Hat Pirates. It would literally make the world One Piece again.
Could be the Poneglyphs told of a person who obviously was not Roger. Gol was not the greedy, selfish type.
Could be that the Poneglyph required something Roger did not have like the Hito Hito no Mi: Nika fruit >!- the same as JoyBoy!<. Maybe it required the power of Poseidon (who wasn't born yet), Pluto, Uranus, or the Noah ship. Possibly all of them.
!Roger was dying by the time he saw the One Piece.!<
There's a lot of reasons he didn't, or couldn't do anything with it.