bounty hunters would go after you after a certain amount of bounty as a world event, they would be categorized by ranks going from C to S. the C rank would have generic skin and use a bandit knife or pistol. already the S could have a more detailed skin and use haki and sword techniques and could be a 2 pistol and haki user.
trapped hunter can be summoned to the crew
when entering a marine base on the top floor of the jail, there may be a bounty hunter from rank C to S. when dropping a cup of sake (already full) the bounty hunter may enter your crew (50% or less of chance depending on rank) and follow you by becoming a nakama.