Since Yamato’s introduction, Oda has been hinting at Yamato being the next crew member, with the likelihood of it happening increasing each chapter she appears in.
Oda has made Yamato a character that is in a personality crisis, she is split between being “Yamato” and the character “Oden”. Being Oden is a major symbolic representation in the manga however, he isn’t just a character, but rather he was also a protector of Wano, a beacon of hope to the land and a saviour, all of which Yamato is trying to replicate. I suspect that by the end of this arc, Yamato will no longer associate herself as “Oden”, but rather accept herself as Yamato.
The reason for Yamato taking on this burden is that she presumed that the son and guardians of Oden had died, therefore there was no one left to protect the land, that is until the Akazaya, Momo and Luffy re-appeared in Wano, thereby removing one reason for Yamato being Oden.Oda loves his parallels, and Yamato reminds me so much of Brook’s story before joining the crew. Both characters want to leave to sails out to sea, but they are trapped on their respective islands until Luffy appears to free them physically, however, in this case, Luffy will also have to free Yamato from her mental bondage of Wano. She is fighting for the Kozuki, but with Momo being aged, will she still need to “fight for them” at the end of this arc, or will they have a protector in grown momo, the Akazaya and a unified Wano?
Oda has deliberately ensured to make us aware of Yamato’s desire to go to sea by her constantly saying the statement in the below picture, and it isn’t ironic that Luffy, the man that is known for breaking shackles and bondages, was the one to free her. I know Oda loves mysteries, but not everything has to be made into a complicated theory, sometimes he places the answer right in front of us, and this time he is saying that Yamato will join the crew, by Luffy being the one to free Wano of Kaido, he is also freeing Yamato from her physical jail and psychological affliction. The introduction of adult Momo, the Yakuza, the re-introduction of the Akazaya, the unification of Wano, ALL of this is leading to the freedom of Yamato. One of the main themes of this arc have been freedom from subjugation, and that is the element for EVERY Straw Hat that has joined the crew.
To wrap this up, Oda is making Yamato a character that is bearing the responsibility of a legend, but she no longer needs to bear that weight, as the glorious Wano of past is ready to be reformed as soon as the subjugator of freedom (Kaido) is defeated by Luffy. Upon this, Oda has already laid the ground work for Yamato to join, from her being freed physically and mentally, to her strong desire to sail out to sea, her alluding to Luffy as being “special”, her connection to Ace, if she is not a Straw Hat by the end of this arc, it’d be puzzling, as Oda does not put this much focus on characters who he doesn’t intend to have joining the crew.
She associates Oden with being a “protector”, hence why the defeat of Kaido is incremental in Yamato accepting herself as Yamato.