Oda had Zoro isolated from the main group off on his own exploration of Wano, his country of birth by all likelihood, meeting Yasuie and learning of its history and seeing it’s miseries, all to further cement his personal ties to the country. To further connect with the populace and the samurai, there’s nothing better than actually slaying a real actual dragon, to replicate what Oden did and at the same time seek and exact justice where it would be most heartfelt: against those who corrupted Wano in the first place, the Koruzumis. In one fell swoop that makes him a true wanoese in the eyes of everyone, worthy of wielding the legendary sword Shushii and carry on Ryuma’s legacy. That’s why I see it makes more sense to fight Kanjuro, whom we now know is not weak as he is the perfect “conjurer”