In the One Piece world, the Straw Hat pirates are constantly increasing their notoriety as every new adventure pushes them to become stronger and a little bit closer to their dream. It takes a certain type of character to fit the role of Nakama, a member of the Straw Hat crew, and with the huge number of fantastic characters that occupy the One Piece universe, it is fun for fans to theorize who may be next to join the crew.
Unfortunately, the Straw Hats are an exclusive bunch; just because a character is well-written or engaging does not mean that they are necessarily Straw Hat material. Currently, in the midst of the Wano Arc, One Piece has put the spotlight on a number of characters that have become a hit with fans, but several of them, for one reason or another, just wouldn't cut it.
Perhaps the character with the biggest fan backing to become the next Straw Hat is the lovable and capable rabbit Mink from Zou. From Zou to Whole Cake Island and all the way to Wano, Carrot has made a case for herself to become the Straw Hat's lookout. There are a few reasons, though, why Carrot joining the crew is unlikely.
While Carrot seems to fit in well with the Straw Hat crew, she has very little motivation to actually join. Every Straw Hat has a goal or dream that they're striving towards. The only mention of a dream that Carrot has made is a desire to sail the seas, but this could easily be fulfilled as she travels temporarily with the Straw Hats. In order to be a true Straw Hat, Carrot would have to bring a little bit more ambition to the table.
Momonosuke has been traveling with the Straw Hats since they met on Punk Hazard. A lot of time has passed in the real world since his introduction and by now, fans probably consider him an honorary Straw Hat for the simple fact that he's been with the crew for so long. Judging by how the story is unfolding in the Wano arc, however, it's becoming clear that Momonosuke would be a horrible choice for the next Straw Hat.
It would be easy to cite Momonosuke's young age, feeble personality or lack of any real combat ability as reasons why he would make for a terrible Straw Hat, but the biggest reason is rooted in the narrative. As the rightful heir of the Kozuki Family, Momonosuke will have big shoes to fill, likely in some position of influence or leadership in Wano after all is said and done. If he were to continue on with the Straw Hats and leave Wano behind, it would defeat the purpose of being sent back in time, finding the other retainers, and fighting so hard to liberate Wano from Kaido in the first place.
A familiar face to One Piece fans, Foxfire Kin'emon seems like a shoo-in as the next Straw Hat. He's quirky, has a strange Devil Fruit ability, and in recent chapters, has proven his mettle as he and the rest of the Nine Red Scabbards do battle with Kaido, an Emperor of the Sea. Between Kin'emon's unique personality and combat ability, fans would be thrilled to see him recant his hatred for pirates and join the Straw Hats on even more journeys.
Unfortunately, Kin'emon has too much tying him to Wano and not enough to the Straw Hats. It's his allegiance to Wano and his commitment to Oden's inherited will that would make him an awful Straw Hat; the crew's dreams, in several ways, play off one another. In Kin'emon's case, there's just no way that he would ever be in service to the dreams of anyone other than the Kozuki clan.
Theories abound when it comes to Yamato: the progeny of Kaido, a former ally of Ace and the self-proclaimed embodiment of Kozuki Oden. Not only are they incredibly powerful, but they have also made clear their desire to leave Wano -- and that it would be possible to do with Luffy. Since their introduction, fans have been convinced that the next Nakama had been revealed.
The Straw Hats wouldn't be strangers to a crew member behaving as if though they're someone else, but Yamato would bring an element to the crew that wouldn't work at all: behaving like someone who has already lived. While the Straw Hats may be able to deal with the likes of Sogeking or O-Soba Mask, bringing a crew member on to not be their true selves feels disingenuous to what it means to be a Straw Hat.
Any Of The Supernovas
Wano has become the meeting place for several members of The Worst Generation, and as a result, alliances are shifting as quickly as the tides. Most recently X Drake, a former member of Kaido's elite Tobiroppo unit, has gone renegade and allied with Luffy and the Straw Hats. It's actions like this that have some fans wondering if there's a possibility of any of the Supernovas becoming an official Straw Hat crew member.
This is likely never going to be the case, however. Most of the Supernovas are captains of their own crew and it's unlikely that any of them would settle for less than a captain's position and fans know the Straw Hat captain can't possibly be replaced. The Supernovas may make fantastic allies, but as crew members, there really isn't a place for any of them aboard the Thousand Sunny.
Speculation of anything One Piece, including who the next crew member will be is something ingrained into the culture of One Piece fandom, and with the amount of characters series creator Eiichiro Oda has introduced, it's a very easy thing to do. Whether the next Straw Hat is a character that has yet to be introduced or is already fighting alongside their future Nakama, one thing's for sure: it takes a truly exceptional individual to become a Straw Hat Pirate.