Zoan Devil Fruits have been charactertized in a different context relative to their Paramecia and Logia counterparts. Rather than just possessing and imbuing power, Zoan Devil Fruits also possess a will of their own and are able to express themselves in ways impossible for the Paramecia and Logia Devil Fruits. Due to having a will, inanimate objects or lifeless mediums possessing no will of their own are able to become host for the Zoan Devil Fruits such as guns, swords, teapots or gaseous slimes. Life is able to be established within these mediums due to the nature of Zoan Devil Fruits. Further, it is also implied that the Zoan Devil Fruits have some power over deciding who consumes them. The Nika Fruit evaded the World Government for over 800 years because it did not want to fall into their hands. With that in mind, could the Okuchi no Makami Fruit have chosen Yamato to be its user for specific reasons?
The history of the Okuchi no Makami is unknown at this point but as per Kaido, the Okuchi no Makami is considered a Guardian Deity of Wano. It is unclear to what extent the people of Wano worship it and whether they still do but Kaido believed he could have used the Okuchi no Makami to continue exploiting the people of Wano concerning running his weapon factories. With Kaido planning to eventually kill Orochi, he sought to have the user of the Okuchi no Makami fill that gap in power. Yamato seems to have some idea about the nature of his Okuchi no Makami Fruit but he rejects Kaido’s methods and interpretation of what the Okuchi no Makami represents to Wano. What Yamato seeks is to liberate Wano and its Samurai and to release them unto the world. The same world Yamato wishes to adventure and explore.
The Okuchi no Makami may very well be a Guardian Deity of Wano but even if that is the case, does a Guardian Deity of Wano need to remain in Wano to continue being a Guardian? We already have evidence in the manga that the answer is no. When Kin’emon and Co. got sent 20 years into the future, they did not need to remain in Wano to fulfill their Guardian Deities role, rather they left and returned. In order to save and protect Wano, they needed allies and as such left Wano to reach Zou. At present a similar scenario is playing out. Wano initially closed its borders to protect itself from an external power and to await the return of Joy Boy. With Luffy being Joy Boy and about to defeat Kaido, Wano will be liberated and its border inevitably opened. Oden wished for this happen and for Wano and the world to support Joy Boy when he returns. Wano’s ultimate victory over the external power they initially closed their borders for over 800 years ago will be determined by Joy Boy. Which in turn means the fate of Wano rest in the journey Luffy is engaged in. By leaving Wano with Luffy and assisting him as Oden wished (Yamato has inherited Oden’s will), Yamato will be performing his role as Guardian Deity of Wano. Sailing with Luffy and performing the role of Guardian Deity are not mutually exclusive. And the home Yamato will return to after the journey will be Wano.
When it comes to the Zoan Devil Fruits having a will, it establishes that the Zoan Devil Fruit has to an extent some choice over who their user is. The Okuchi no Makami Fruit chose Yamato. Not the Samurai. Not the Beast Pirates. Not an average civilian. But Yamato. A child who idolized Oden and wished to adventure and sail the seas. Why? Why Yamato? If the Okuchi no Makami Fruit was concerned with the protection of Wano, there would have been better options than a child of Kaido who was imprisoned on Onigashima unable leave. Zoan Devil Fruits may have a will but they cannot see into the future and know with certainty what will happen. It was not guaranteed that Yamato would continue to challenge their father for 20 years. The likely scenario was Yamato breaking down from despair, hopelessness and suffering to become a soldier of Kaido. Rather than some future event that hasn’t happened motivating the Zoan Devil Fruit, what appears more likely is the current disposition of the individual being the focal rationale behind the Zoan Devil Fruit choosing the person. Did the Okuchi no Makami react to Yamato’s adventurous and romantic spirit and his desire to carry on Oden’s will?
When exactly did Yamato consume the Devil Fruit? Considering Yamato did not recognise the Fruit as a Devil Fruit and realise consuming it would remove his ability to swim, it seems likely Yamato consumed the Devil Fruit at a young age. When Yamato aged, he was more capable of defending and providing for himself. For Yamato to have reached a point where he was hungry that he would eat something he didn’t fully recognise, it conveys an age where he was more vulnerable i.e. when he was younger. If I were to guess why the Okuchi no Makami chose Yamato, I would say it was Yamato’s desire to be free and to see the world that resonated with the Okuchi no Makami as that would have been the main dreams inspiring a younger Yamato. While the Okuchi no Makami may be a Guardian Deity of Wano, it was consumed by someone who wished to leave Wano and adventure on the seas. Yamato possessing the Okuchi no Makami Fruit does not by itself equate to inheriting a role limited to just Wano Kuni.