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.