There were two comments in Act 2 about Enma:
-Enma is the sword that gave Kaido his only and only one scar
-Enma it's still not black, and that depending on Zoro it could become black
The narrative of the first comment was already fulfilled since that Zoro replicated Oden's feat of scarring Kaido, despite many fans saying that it was a superfluous comment and that it wasn't implying that Zoro was going to scar him
The second point is still open, but it will covered before Wano's end as all the Enma blackening plot makes sense only if it ends in Wano, because otherwise Wano's people wouldn't be able to witness the birth of the new sword god directly
As per Zkk I do believe that it is strictly tied to Enma blackening, because I see Zkk as the natural converging point of all the subplots related to Zoro