Thirty years ago, Marco was among the Whitebeard Pirates when they arrived for the first time at Wano Country. He was 15 years old at that time and was an apprentice on the ship.
At an unknown point in his earlier pirate days, Kaido started apprenticing in the fabled Rocks Pirates, which also counted Charlotte Linlin and Edward Newgate among its ranks.
In the Zou arc, it is mentioned that Nekomamushi and Inuarashi were just apprentices at the Moby Dick and at the Oro Jackson (also mentions Shanks and Buggy) and that got me thinking why should the Straw Hats not get an apprentice too.
The crew is filled with strong, good people, who could very well nurture a young talent, and with that not only gaining a new strong combatant but also leaving a legacy to the future.
What would be of the One Piece if Roger hadn’t taught Shanks? We wouldn’t have Luffy.
Now, we may have some candidates to be new member of the crew as an apprentice: Tama, who gets along with Luffy, has a useful devil fruit and is being well portrayed.
Furthermore when Ace visited Wano, Tama formed a bond with and had faith in him. Tama wanted to go out to sea with Ace, but he told her that she was too young at the time. He promised that he would take her with him if she became a kunoichi by the time he returned and Tama waited for him since.
What if Luffy keeps Ace’s promise and takes O-Tama with him?
Also maybe Toko, the daughter of Shimotsuki Yasuie.
Anyway I don’t think it’s any of those two nor anyone else but Momonosuke.
He’s perfect for this role, he has similarities to Luffy such as being strong willed and having the ability to hear the Voices of All Things. He has the ability to communicate with the giant elephant Zunisha, being capable of seeing through its eyes and giving it orders.
Also his father left Wano to sail as a pirate and he has his father as a role model, plus Luffy has clear similarities to Gol D. Roger, and Oden sailed with Roger.
Also, in the end of the arc, Wano will need a new Shogun, but Momonosuke is too young, he can’t rule a powerful nation that will most likely by the end of the arc, open its borders. And he has potential as a fighter as well, with his Dragon Zoan devil fruit (even if it’s fake, it’s made by Vegapunk himself, and we still haven’t seen its potential).
To wrap it up, Momonosuke will join the crew after Wano as an apprentice and will sail to the end of the story with the Straw Hat Pirates. There are similarities between him and his father, Luffy and Roger, and most importantly between those two pairs, supporting the idea that a similar path might be taken by Oda, showing Luffy mentoring Momonosuke, which is also a great growth experience for Luffy.
*Theory by Pedro5822
