Usopp lies come to fruition,so since he needs to someday lift an unlifatble hammer. Ragnir fits this.
Now as for ZKK. Even though I never believed it for some reasons(E.x outshining Luffy in the arc when he would be declared Yonko,SH killing on panel etc. )
Oda purposely planted some seeds here and there.
-The defacto second coming of Ryuma
-He was created as the deuteragonist to Luffy because he was like Ryuma.
-The outsider hero who return the blade of the God of the blade.
What Ryuma did?
Slay a Dragon above the flower capital
ZOro inherits Enma and so through the sword inherits the will of both Hiyori(Wanted to kill Orochi herself& Oda didn't even let Zoro slice a single head of his) and Oden.
Yasu's speech where Oda uses the common trope of panning to the prophesized person with the speech of somebody else being spoken/written
Oda knows though that having Zoro do some things will end up having outshine Luffy so he writes stuff to keep Zoro's fans engaged but in the end fuck him over.
Zoro:I will round up the samurais=Luffy does
Zoro:I will avenge Yasu=Neither Orochi nor Kaido fell on his hands,just a mere underling
Zoro creates a new permanent scar on Kaido,only the second after Oden=Oda has Zoro downplay it himself by commenting the knocking him to the ground dialogue and him drawing the scabbards merely knocking Kaido to the ground as a more grandiose feat achieving what Zoro wished,even though it was actuallya shittier feat..
Oda tried to gaslit with Zoro,he did nothing short of that with Usopp. Just a meme theory by fans according to previous Usopp lies coming to fruition.