A casino in the new world with odds on who will find One Piece.
Basically, if some random citizen wants to bet his paycheck on who will become the pirate king, he should be able to. As Luffy moves forward, his odds improve and the amount of money that can be made by betting on him goes down. Half the island tore up tickets they had saved when Whitebeard died. I think such a system would be interesting by itself, but ultimately the idea appeals to me for a different reason.
That reason is contrast. The bounty system is great, and that's partially because it's an imperfect system (unlike stupid Douriki). Bounty numbers are decided by people who make mistakes. The only thing that could make it better is an alternate, equally flawed system to go with it. Bounties don't tell the whole story, but typically the reader will assume that the person with the higher bounty is more likely to win the fight. However, if one person has a higher bounty, and the other person has better odds to become pirate king, the matchup is immediately more interesting to the reader.
That's a really cool idea, though one questions whether such betting would be legal (bounties get around because the WG wants them to).