We’ve had some good joke predictions like Caribou and Caesar who will join the Straw Hats after Wano has wrapped up. We also can’t deny Tama who’s been a persistent darkhorse candidate since the arc began. But let’s be real here, it’s all gonna come down to Carrot and Yamato.
In my opinion, the reason why they are such prime candidates is something that doesn’t come down to popularity, fight records or even panel time. It all comes down to their inherited will and how they connect to the over-arching plot of Laugh Tale, Joy Boy and Luffy becoming King of the Pirates. For this reason, I believe the next Straw Hats will be both Carrot and Yamato.
Now, inherited will isn’t always a guarantee that someone will be a Straw Hat, but the inherited will of Carrot and Yamato are special cases. The wills they inherited are from characters who are tied directly to Gol D. Roger, Laugh Tale, the poneglyphs, ect. Pretty much all the important stuff for the endgame.
Carrot inherited the will of Pedro
Pedro’s dream was to see the Dawn of the World. We still don’t know the specifics of what that is, but judging from Pedro’s words and actions, it must have involved sailing with and assisting the future Pirate King. He was unable to get his chance to sail with Roger as a young Mink, but Roger assured him that his time will come.
Whatever the Dawn is, Pedro was adamant to be a witness to it. He formed an expedition-party-turned-pirate-crew called the Nox Pirates for the sole purpose of seeking out a Road Poneglyph that eventually found its way into Big Mom’s clutches. After failing to steal the Poneglyph and losing his friend Zepo in the process, Pedro ripped out his own eye as a way to convince Big Mom that he needed to survive to see the Dawn.
Considering the lengths Pedro was willing to go to, either in his own survival or tracking down the Road Poneglyphs (the only method to get to Laugh Tale), we can infer that whatever the Dawn might be, the answer lies on Laugh Tale.
Fast-forward to the Whole Cake Island arc. Pedro was certain the Straw Hats hold the key to ushering in the Dawn of the World because of how they saved Zou from destruction twice. But since he knew he wasn’t going to make it out of Totto Land alive, he told Carrot about it before he sacrificed himself so that the Straw Hats could be freed from being trapped in candy and be able to make their way out of Big Mom’s territory. This event was him passing down his will to witness the Dawn of the World to his pupil, Carrot.
Yamato inherited the will of Kozuki Oden
After witnessing the Legendary Hour, Yamato sought to emulate Oden for his bravery and loyalty to his retainers. They voluntarily expressed the desire to carry on Oden’s legacy. Not just in finishing what he started by defeating her father and opening up the borders of Wano, but by also being free to go out to sea and travel with the future Pirate King like her idol did.
Yamato also inherited the will of the samurai of Wano who were imprisoned with them. They were the people who taught Yamato how to read Oden’s journal in the first place. And once they broke out of their prison to their demise, the samurai made it clear that Yamato’s survival was essential to the battle 20 years later.
It should also be noted that, in between those years, Yamato learned about the Dawn of the World as she states to Momonosuke that he is an integral part of it.
Carrot and Yamato respective inherited wills are not only similar, but also complimentary. Both of them wish to see the Dawn of the World, but also they don’t know what exactly that entails. Carrot only knows about the Dawn because Pedro told her how important it was to both the Mink tribe and the Kozuki clan that Luffy and the rest of the Straw Hats make it out of Totto Land.
It’s implied that Yamato might have more information about what the Dawn is considering how they used it as an example to motivate Momonosuke, but they also don’t have the full picture because there are pages missing from the journal where Oden tore them out. Therefore, it would make sense for them to realize that ensuring that Luffy and his crewsmakes it to Laugh Tale to bring forth the Dawn of the World because that’s the most logical course of action parsed from the information they have.
Also, the islands of Fishman Island, Zou and Wano Country figure heavily into the lore about Joy Boy and the poneglyphs. We have Noah and Joy Boy’s apology letter on Fishman Island, Joy Boy’s old companion carrying Zou on his back, and Wano being the place where the Kozuki stonemasons crafted the poneglyphs during the Void Century.
Since we already have a representative from Fishman Island on the crew, wouldn’t it also make sense to have a representative from Zou and Wano also on the crew?
Theory by KaiserBeamz (https://www.reddit.com/user/KaiserBeamz)