The basic point of this theory is that Yamato will become the Shogun of Wano and Momonosuke will go out at sea.
1. Momonosuke has doubts of being a leader of the Kozuki Clan. He realizes that people see him not as Momonosuke but as Oden’s heir. Furthermore Momonosuke needs to get more experience and growing up to even effectively lead a country.
2. Momonosuke has been away for 20 years. How could Momonosuke lead a country that he has little knowledge of?
3. Yamato seems to have been influenced by Oden by the way he looked. Perhaps he was at Oden’s execution and his ideals stuck with him and influenced him.
4. Let’s assume that Oden’s ideals were with Yamato the whole time. This would be a good reason why Kaido had to send people to find him. Yamato has been trying to help the people of Wano, while Momonosuke and the Scabbards were gone. This would mean that Yamato knows the people inside and out while Momo does not.
So at the end of the Wano arc, Momonosuke would give the position of Shogun to Yamato as he still has little to no experience to the current would.
Now here’s the symbolism:
Yamato looks like Oden as if he is the will in a physical form to help the people of Wano.
This would also break the “Tradition” of Wano that have left it secluded and closed off to the world. What better way to change Wano for the leader to be someone who loved the country and for their merits rather than the blood in their veins? Because let’s be honest, as good as Momonosuke is, he’s just a figurehead in this rebellion. He didn’t plan the recruitment, he didn’t look for the Scabbards and he’s not even leading the charge.
Now Momonosuke would not be the Shogun but would experience the world for his own like his father Oden.
*Theory by KillaThing