Bounty Hunters should've been a bigger part of the story, imo. I've advocated that Mihawk could've been one to help establish them as a real thing in the series. Bounties, as they are, serve their role as hype tools for antagonists and a post arc reward for the Straw Hats, but their implementation would feel more natural and add to the world building if they served an extra layer of purpose.
Villains have bounty > Straw Hats get new bounties because said Villains > Straw Hats get chased by new Villains
Although, you could argue the Marines sort of alleviate this issue because, while their priority isn't the bounty attached to the pirate, the larger amount means they become a higher priority for the military, more often than not.