Miss Fortune is specifically a bounty hunter, she may have a pirate hat but she is not a pirate herself. She may be a captain of a ship, but her occupation does not include anywhere she's a pirate. This is very plausible, simply because during the Golden Age of pirates, their were pirates who were captains, but there were also just captains who weren't pirates, many of them even hunting pirates (akin to GP and MF relationship). Graves is a mercenary, a thief, and apparently a wanted man in all the realms. Twisted Fate is someone who is sharp with cards as well as being a thief.
Gangplank is the <i><b>only</i></b> champion in League of Legends who's occupation specifically states that he is a Pirate, as well as being a captain (well former). All I think is that they need to atleast add <b>one</b> more champion who's occupation specifically states he's a pirate (he doesn't have to be a captain, but he must be a pirate), embodying the many other aspects of pirates that GP does not embody.