It's going to be one piece of gold, and you realize that the Pirate King was never the Pirate King because of his wealth, but because of his awesome attitude. Luffy will realize this and be like, yeah, now I'm pirate king.
Also, one piece = one peace... the great wars between the world government and the pirates will end when the new Pirate King reigns.
That's what I've thought since like, chapter five. I have no idea if it'll be true.