When Monkey D. Luffy first left Foosha Village in his little dingy to set out to sea, he decided that his pirate crew should have 10 members. Since then, we've watched this rubber man in a dingy grow to become a capable pirate captain of a crew with one of the highest bounties in the entire world. And now with the Fishman, Jimbei (also known as Jinbe/Jinbei), finally an official crewmember after so long, fans thought there wasn't much room left for the Straw Hats to grow. But that might not be the case.
In the most recent edition of One Piece magazine, the translation of Luffy's original line reveals that he (the captain) is not included in the specified 10 crewmates, meaning that the Straw Hats still have one crewmate left to recruit. Who could that be?
The important thing to note first is the role. Everyone in the Straw Hat pirates has a job they fulfill: Nami's the navigator, Sanji's the cook, Chopper's the doctor, etc.. Jimbei, the most recent member, has been given the job of the helmsman, the person who steers the ship. With his knowledge of ocean tides as a Fishman and years of experience as a pirate, there's no job more fitting for The First Son of the Sea. So, what role on the crew is left unfulfilled and who are the most likely candidates to fulfill that role?
Taking a look at a proper breakdown of a pirate crew's roles, you can see the Straw Hats follow a traditional model. Carpenter fits Franky, as the shipwright; the sniper, Usopp, can also be referred to as the Gunner and Nami, the navigator, would fit the role of Sea Artist. From that list, the key roles that appear to be missing on the Straw Hats are the Quartermaster, Master and Striker. There's one fan-favorite character who could fit one of these roles, and that's the rabbit Mink, Carrot.
Carrot first appeared in the Zou Arc, and then stowed away on the Sunny as Luffy, Nami, Brook and Chopper took off for Whole Cake Island. Since then, she has been a beloved ally of the Straw Hats and currently faces Kaido's crew alongside the crew at Onigashima in the manga. She's an adept fighter, trained directly under Pedro, and skilled enough to be able to unleash her Sulong form against Big Mom's crew. Minks also possess a great sense of smell and hearing due to their animalistic qualities. With that in mind, Carrot would fit the role of Striker -- hunters who gather food for the crew. The only problem is that, culturally, Minks don't eat anything with fur so it's more a question of if Carrot would hunt, rather than if she could. She might even make for a better Scout, being able to warn the crew of danger before it even happens.
There's a very recent character that may end up being the final addition. During the Onigashima Invasion, Luffy was saved by Yamato, Kaido's only son. Assigned female at birth but identifying as male, this honorable warrior idolizes the hero of Wano, Kozuki Oden, so much that he's willing to betray his father and help Luffy defeat him. This infuriates Kaido enough to punish Yamato, even attaching bracelets with bombs in them so he would never leave his sight. Yamato has more than enough reason to join up with Luffy and it would be very fitting for the son of Luffy's current enemy to help the Straw Hat defeat him.
The only thing stopping this at the moment is how little we know about Yamato. He's a skilled fighter and possesses his father's monstrous strength, demonstrated by him wielding a Kanabo club. Assuming Yamato was raised to be Kaido's successor, it's likely he was taught to lead and command as a captain. This is only a theory, but that could potentially make him a fit the Quartermaster role for the Straw Hats, responsible for divvying up treasure and keeping track of goods and rations.
However, there is one other close ally of the Straw Hats that may be more suited to be Quartermaster -- an old friend from Alabasta by the name of Nefertari Vivi.
The Straw Hats met Vivi when they first entered the Grand Line and agreed to return her to her home country of Alabasta to stop Sir Crocodile from manipulating her people into civil war. Throughout the few arcs they spent together, Vivi became close friends with the Straw Hats -- so much so, in fact, that it was incredibly difficult for them to say goodbye to her when they left. Vivi stayed behind to look after her people as the future ruler of Alabasta, and over the course of these last two years, she's grown a lot as a leader, even attending the World Council meeting alongside her father. Her skills as a diplomat and conviction for what is fair and just would make her an excellent Quartermaster for the Straw Hat pirates if she were ever to return.
Knowing that there's just one more Nakama left to join the Straw Hats only underlines the fact that, over 15 years after it began, One Piece's ending is fast approaching. Whether the 10th member is Carrot, Yamato, Vivi or someone we've yet to meet, they must possess the strong will to fight, a dedication to a skill they alone possess, but most importantly, a dream they'll fight until the very end to see become a reality. At the end of the day, these are the qualities that make a Straw Hat pirate.