given the shonen pattern, we should all agree that teach and akainu > kaido, but we all know that hody jones came after enel, so OP doesn’t follow the same logic. I think that kaido will be the strongest enemy luffy will ever face, maybe right after imu. The difficulties vs akainu and teach will probably be more related to the personal conflict involved (ace).
kaido won’t fall for luffy’s punch, but will be 100% be defeated by luffy. Assuming that, zoro killing kaido may be a possibility, but would it really add much to zoro’s character? zoro killing an already defeated and maybe even passed out kaido? i bet this would be some bad writing. That’s why i’m against ZKK