What if Luffy is not only gathering crew anymore, but alliance like whitebeard had in the war. So individuals like Jimbei, Hancock, Crocodile, Marco ect. may not become crew mates but allied pirates for the future war against the world government. I beleive that the elders know of the ancient history and the full story of the Ds, because they are direct decents of the original men or woman who faught the world government; or somehow carry theyre will. So i do not believe one piece is treasure it itself but it is the ability to unite all the carriers of the will of D in one piece to fight against the world government.