Kozuki Oden had TWO wishes. One to open the borders of Wano and the second to assist the figure the world had been waiting for over the span of hundreds of years i.e. Luffy. It is pretty apparent at this point that Yamato has inherited the will of Oden and is currently fighting to see Wano Kuni liberated from Kaido and subsequently have the nation’s borders opened.
The battle which Oden mentioned would take place 20 years after his death was not just the battle against Kaido but also the battle against the World Government, the enemy which the Kozuki closed Wano Kuni’s borders to defend against. Yamato wants to fight on Luffy’s side. And such a desire will not only entail the battle against the Beast Pirates but also the inevitable battle Wano and those nations that align with their beliefs will have against the World Government when they become the target. Yamato’s wish to get stronger as he sails the seas before fighting in the war will manifest as they join the Straw Hat Pirates.
When Oden mentioned that the Kozuki clan closed Wano to the rest of the world to protect against a great external power, it is implied that such a force would have reason to attack Wano if they had remained opened. The great external power Oden alluded to is without question the World Government. Once the borders of Wano are open, the World Government will once again target Wano Kuni but the response from them won’t be immediate, rather, it will be a careful and weighted response. Wano will gain support in their opposition to the World Government by other nations and sovereignties that don’t agree with the World Government. The battle the Marines and the World Government has with Wano and those that side with them will be the war that climaxes One Piece. The Straw Hat Pirates will be part of this war and you can bet that Yamato will to after sailing with the Straw Hat Pirates and getting stronger. The wish Yamato mentioned to the Samurai (Daimyo’s) twenty years ago will manifest.
Yamato has been in possession of Oden’s journal for two decades but only came to realise the importance of the words contained in the pages after a certain event two years ago. Yes, prior to Ace’s death, Yamato was not aware of the importance Luffy held in regards to the details of Oden’s journal. This is why when Yamato first met Ace, he wished to leave aboard their ship because at that time he wished to be free and sail the seas going on adventures (which he still wishes for). If Wano wasn’t chained by Kaido/Orochi and Yamato wasn’t bound by the explosive cuffs, he would have got aboard Ace’s ship. Fast forward ~2 years when Yamato learned about Ace’s death, he also learned about Luffy and recalled what Ace told him years prior. Such memories about Luffy and Luffy’s dreams connected to the writings in Oden’s journal. Yamato realised at that time the person the world had been waiting on, the person Oden had been waiting for, was Monkey D. Luffy. This is the moment when Yamato knew what he had to do – what he wanted to do.
Not only did Luffy, Zoro, Nami, Usopp, Sanji, Chopper, Robin, Franky, Brook, Jinbe and Sabo have their ‘aha-‘ moment when Ace died but so did Yamato. Within the grief and sorrow, they found a path forward.
Ace’s death allowed Yamato to make the connection between Luffy and Oden’s journal (Yamato’s “bible”). And this is why Yamato’s dreams have evolved from what they were when he met Ace ~4 years ago (sailing the seas, going on adventures and opening Wano’s borders) to what what he is expressing now on Onigashima during this arc. Yamato’s dream is not only to help open Wano’s borders but it is also to assist Luffy who is the person that will shake the world to its core creating waves of change. Yamato now understands how Luffy fits into Oden’s SECOND wish which will be a role he will bear on his shoulders going forward. Yamato will join the crew and help them see through the unfulfilled work Joy Boy and Roger have left incomplete.
No other Straw Hat Pirate has been connected to Joy Boy and the mission they left behind in terms of their awareness. Not even Robin. She only learned about Joy Boy’s existence a month ago in One Piece time and is still not aware of Joy Boy’s relevance to the history of the world. Joy Boy was only revealed during the Fish-Man Island Arc AFTER the time skip. It is almost as if Oda had begun at that time setting the foundations for a new character (nakama) to fill a key story role aboard the Thousand Sunny that no current members at that time fulfilled. Yamato will assist in providing clarity to Robin about ‘Joy Boy’ and their place in history.
Alongside his admiration for Oden, Yamato is also expressing their own desires. Yamato will help Luffy liberate Wano Kuni and then the world. Fitting for someone who has been imprisoned and robbed of their freedom for the majority of their life. A freedom Luffy and Momonosuke will assist Yamato in reclaiming from Kaido.
Yamato’s interest in Joy Boy and the Dawn aren’t inconsequential but rather a driving force behind his intentions and actions. Yamato is aware of Momonosuke and Luffy’s importance to the future of the world and the “Dawn”. Ever since Oden’s hour long execution, Yamato has sought to be Oden. And after reading Oden’s journal, he has wished to fulfill Oden’s will. And within that wish he has combined his own desires – being free, sailing the seas and having adventures. Luffy is too important to Yamato and his dreams. Knowledge of Luffy’s dream brought joy and tears to Yamato. The only way Yamato won’t be sailing aboard the Thousand Sunny is if Luffy rejects him and that ain’t happening. In regards to Momonosuke, he won’t be the one to rob Yamato of following their dream.
Oden is a source strength for Yamato. While Yamato’s need to imitate him may decrease going forward, the admiration and inspiration he feels and receives from Oden will remain. Oden’s wishes will be fulfilled and Yamato will be the one to see it through via their adventure and journey to Laugh Tale (and beyond). Along the journey, Yamato will also learn more about their family origins and history. Kaido is a massive mystery and his story within Wano Kuni won’t provide us with all the answers to the questions we have.
As for Yamato’s ship role, i wonder if Oda will just settle on Samurai for Yamato. He did want to be one and everyone else around the world views warriors from the nation as Samurai. Or does Oden constitute the highest level of Samurai to Yamato? I wonder if Samurai conflicts with Zoro’s role. Yamato could be the chronicle of the crew so that each of the member’s story is recorded within text. I’m still waiting to see if Yamato has stonemason skills O.o.