The Straw Hat Pirates are an incredibly powerful pirate crew who have gone from strength to strength over the years. Led by Monkey D. Luffy, this pirate group is now considered to be one of the Four Emperor crews of the One Piece world in the Final Saga, which goes to show how strong they truly are.
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However, there are some members of the Straw Hat Pirates who still need a significant strength boost, and a Devil Fruit would be perfect for them. Four members of the crew already have powerful Devil Fruit abilities, but there certainly needs to be an addition if they want to compete with the top crews of the world.
Prince Loki
Loki is the Accursed Prince of Elbaf and one of the most important characters in the Final Saga of One Piece. He is known to be a very powerful individual, as strong as the Yonko. Loki is known to wield a legendary Devil Fruit power, the true nature of which remains unknown for now.
Luffy has already asked Loki to join the Straw Hat Pirates, and at the moment, he remains the frontrunner for the position of the last Straw Hat. If Loki joins the Straw Hat Pirates, he is undoubtedly going to be one of their strongest Devil Fruit users.
Usopp
Usopp is the sharpshooter of the Straw Hat Pirates and one of the East Blue Five members whom Luffy sailed to the Grand Line with. Usopp is powerful right now, being strong enough to compete with some high-ranking pirates.
However, it is no surprise to know that compared to the top tiers of the Straw Hat crew, Usopp is very underwhelming. He needs to get a power-up in one way or another, and a Devil Fruit is perfect. However, Usopp eating the Devil Fruit himself doesn't make much sense. Instead, it is his weapon, Kuro Kabuto, that can eat the Devil Fruit and make him stronger.
Given that only Zoan Devil Fruits can be fed to inanimate objects, it is highly likely that Usopp's Kabuto could eat a Zoan-type Devil Fruit that would allow him to bring out the next level of strength as a sharpshooter.
Nami
Nami is a very shrewd member of the Straw Hat Pirates. Among all the crew members, she is easily one of the smartest. However, when it comes to combat, Nami is quite easily one of the worst ones. She has Zeus at her disposal, but against the strong enemies, even that is not enough.
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This is precisely why Nami needs a new Devil Fruit as well. She could even feed the Devil Fruit to her Climatact. Either way, her strength would increase significantly. Ideally, something that allows her to manipulate elements of weather would be perfect for Nami.
Yamato
Just like Loki, Yamato is not a member of the Straw Hat Pirates just yet. However, at one point, Yamato was the prime candidate for being the last Straw Hat. At the end of the Wano arc, Yamato decided to stay behind to guard the country from external threats, but Luffy always kept the door open for them.
It is no surprise that Luffy would welcome Yamato with open arms and once the Great War begins, Yamato might ally with the Straw Hat Pirates one last time. Fans already know that they have a very powerful Okuchi no Makami Mythical Zoan Devil Fruit and it is quite easily one of the strongest in One Piece. This Devil Fruit allowed Yamato to compete even with Kaido in combat. Yamato would easily be one of Luffy's strongest underlings, and in the Great War, if Yamato did join, they would become an invaluable asset.
Vivi
Princess Vivi of Arabasta is one of the prime candidates for the spot of the last Straw Hat. Technically, Vivi is already a Straw Hat and even Oda himself considers her to be part of the crew. Vivi can rejoin the Straw Hat Pirates and will most likely do so eventually. However, given her level of strength, she would feel out of place. This is precisely why Vivi needs a powerful Devil Fruit to boost her strength significantly, and that is what would make her an asset to the group.
Vivi's importance in the Final Saga of One Piece is paramount. Fans already know that Imu wants to get their hands on her. More than that, it has also been made clear that the Straw Hat Pirates are eagerly waiting to reunite with her. When she does rejoin the crew, having a Devil Fruit at her disposal would help her out tremendously.