Enma is one of the two blades of Kozuki Oden. Oden willed his blades to his two children Hiyori and Momonosuke. Zoro literally just got Enma in a trade with Hiyori for Shusui the legendary treasure of Wano.
Thing is after recent events Momonoske may be aging soon and as the new ruler of Wano (if all goes well). I just can’t see Zoro taking Momonosuke’s fathers birthright blade with him.
I think Hitetsu will offer Nidai as a trade for Enma after recognizing Zoro’s lineage.
For two reasons, the first of which is foreshadowing. Zoro saw Luffy wielding Nidai and was interested in seeing the blade. Luffy immediately shut it down and denied Zoro the opportunity. He then returned the blade to Hitetsu. Zoro’s interest in the blade instituted a Chekhov’s gun situation. He couldn’t see it because he will eventually wield it.
Secondly, Enma is a powerful sword. But it’s relevance lies in its innate ability to draw haki out of its user causing a massive strike. But in wielding Enma you are innately training yourself in haki control.
I believe after mastering the blade, Zoro will no longer need Enma for its ability and will only keep it for its grade level. Which would be a waste as Wano new ruler will have a need to use Enma to learn the same skill Zoro has mastered and protect his newly opened country.