I want it to be against either King or Kaido, but hear me out first! i know it sounds bad.
I think Enma is already half way there with the years being used by Oden, so Zoro would just finish the deal by completing that blade's inherited will, either ZKK or just helping the borders of wano open up by defeating King.
I also think Zoro leaves Wano with the 3 shimotsuki blades: Shusui (given to him after his lineage is revealed), Enma and Wado.
Then he turns Wado into a Black Blade when he completes the inherited will of Kuina by becoming the strongest against Mihawk.
and then Zoro ends the story with 3 black blades.
but the one he completely turned from start to finish was Wado.
Do you guys hate this idea that much?