Like many others, I believe that the Wano arc will be special for Zoro just like the Whole Cake Island arc was special for Sanji.
Sanji created the Wedding Cake that was needed to calm Big Mom down. This cake was hyped up to be an absolute masterpiece and it was created under incredibly hard conditions, thus showing off Sanji’s legendary abilities as a cook.
If Oda will do something similar for Zoro in the Wano arc, then there must be a task big enough that Zoro’s abilities as a swordman will be considered legendary if he is able to accomplish it. Being able to cut Kaido would certainly fit that description since Kaido was hyped up to be virtually invulnerable. This is illustrated by the fact that every single execution weapon broke when being used on him. Since Zoro likes to tell us that “there’s nothing he can’t cut”, Kaido’s invulnerable body seems like the perfect challenge for him.
I certainly don’t think that Zoro will defeat Kaido. I don’t even think that they’ll fight. Rather, I believe that Kaido (perhaps while being drunk) will accept a challenge by Zoro according to which he has to “leave Wano / spare the Straw Hats’ lives / don’t kill civillians” if Zoro can cut him. Then, Zoro will use his completed set of Kitetsu blades to cut Kaido, thus saving the day in a similar way Sanji did.
*Theory by OscarTheFountain
