That's why I say that the Enma blackening plot should end in Wano, because the direct comparison set by Oda (through Hitetsu) between Zoro and Oden holds more weight if such feat is accomplished in Wano and not after.
Oden the previous holder of Enma who couldn't forge a black blade
Zoro the current holder of Enma who is 100% destined to forge Enma black
There is no better settings other than doing it in front of Wano's people, so that they can witness directly the birth of a new black blade centuries after Ryuma's case.
It just fits thematically