Another candidate could be Toko, the daughter of Shimotsuki Yasuie, the former daimyo of Hakumai, and a member of the Shimotsuki clan.
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.
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.
Hiyori and the remanig Scabbards will rule Wano waiting for Momonosuke to come back and take his place as Shogun.
*Theory by Pedro5822