Given the recent chapter I’ve tried to formulate how I suspect Oda will write the progression of Zoro in his journey as a swordsman.
Hiyori has offered to replace Shusui with Enma, a Meito sword that was once wielded by Kozuki Oden.
What comes to mind to me is this idea of what Shusui represented to Zoro’s progression.
No doubt that at Thriller Bark he was weilding a blade superior to that of the Skilled Grade Yubashiri and thus it provided a challenge for him to progress. However, this idea that any blade can become a black blade leads me to think that at one time, likely in Ryuma’s posession, Shusui was either a Skilled Blade, a common Wazamono, or perhaps even an ordinary blade. However after travelling by his side the blade eventually became black and thus increased in grade.
From this I drew the idea that once a blade is forged it can no longer progress beyond the scope of its grade. In other words, beyond Armament Haki, a sword has no other progression and thus Shusui has made its final journey with Zoro. Now with that said, Enma to me could easily be a superior grade weapon, the greatest Zoro has yet to posess, but this seems sort of a lack of opportunity on Oda’s to emphasize the progression as Zoro as swordsman. By this I mean that if Zoro were to posess Enma, a blade which I think is a regular Great grade blade, he would have the chance to turn it into a black blade.
More importantly, if Zoro were to posess the Nidai Kitetsu, he could very well do the same with it along with the Wado Ichimonji giving him 3 Superior Grade weapons as an end result.
Why this is important to his progression is his final bout with Mihawk. Mihawk’s blade Yoru, if we follow this logic, was probably a Superior Grade before Mihawk made it a black blade giving it a special position even among the Superior Grade blades.
In this vein of thought, if a swordsman hoped to defeat the World’s Greatest Swordsman weilding the Greatest Sword in the World, they would have to hone their skills enough to either come into posession of one of the other 11 Superior Grade blades or turn one of the 21 Great Grade blades into a black one. Only then would they hope to survive in a serious match against Mihawk and not have their blade break.
In other words there are only 32 blades that could achieve it in the One Piece World and I predict that by the end of Wano Zoro will posess three Great Blades which will become Black Superior Grade Blades by the time him and Mihawk fight.
*Theory by AWorldWithoutString