Just speculating about some thoughts I have. Haven’t read any speculation like this before but if there is one or several I apologize. Well, let’s get to the point.
I think the one that will defeat Kaido is none other than Zorro.
For one simple reason. Ryuma was known as the most legendary Swordsman and known as the only man capable of killing a dragon. This reminded me of the moment Kaido came from the sky out of nowhere (except the clouds changing). For all we know the same might have happened with Ryuma. It might be that the dragon Ryuma fought was not fully evolved but Kaido certainly is. We also know Kaido’s skin is probably the hardest thing to break/cut. If there is anyone that can overcome this it would be Zorro. Making him stronger than Ryuma.
It might not be that he is going to do it all alone but I think the final blow will be by the hands (swords) of Zorro.
I wouldn’t be surprised if this would make the people of Wano want him to be the leader of the Samurai instead of Momo. But Zorro not wanting to be ‘stuck’ decides to let Momo ‘defeat’ him. This would also mean character development because I don’t think Zorro would want anyone to win from him in any way.
Just running some thoughts here but the main point is Zorro will defeat Kaido because Ryuma did it in the past and Oda likes to let history repeat.
Hope you enjoyed this small speculation.