During the Wano arc we learned that Shusui is a ‘black blade’, and that swords become black blades through the efforts of their owners.
Previously Mihawk had stated that Yoru was the world’s strongest ‘black blade’, and added that ‘any blade can become a black blade’. We all assumed this was just Mihawk referencing Armament Haki, but we now know that becoming a black blade is an intrinsic property of the sword.
So my theory is that transforming a sword into a ‘black blade’ actually increases the grade of the sword. In this sense, Shusui might have become an O Wazamono grade sword only after becoming a black blade. If that’s true, then maybe Zoro can transform the Wado Ichimonji into a black blade, making it a Saijo O Wazamono eventually?
This would resolve what I perceive to be a real problem with the Wado Ichimonji because other swords are outclassing it. Shusui is already Zoro’s “go-to” sword for his Ittoryu (one sword style). Legendary sword Enma has appeared and will probably be stronger than the Wado Ichimonji. And yet Zoro swore to wield this sword and become the world’s greatest swordsman. If he does that while primarily relying on other swords, it cheapens his oath.
All of this is solved if Zoro transforms the Wado Ichimonji into a black blade, making it his strongest sword (even stronger than some other supreme grade swords).
*Theory by Alpha_ii_Omega