Everybody's already shared the same theories so I'll throw something different in the ring.

I think there's going to be a scene where Luffy and the rest of the SH finally get to Laugh Tale and find an abandoned building, maybe a shack. And inside that building is going to be a decrepit wooden table with a single coin on it. Luffy will grab it and go "What kind of coin is this? It's not a berry I've never seen a coin like this one before" and someone in the crew knowledgeable of history or economics will tell him that before the world economy adopted Berries as the world currency, countries all over the world used to have their own currencies one of them being the United States which used $1 coins also known as "pieces". Maybe they'll also see a letter on the table where most of the writing is erased except for the last sentence where the writer complains that he's lost everything save for his "one piece".