After chucking Luffy off the roof, Kaido very interestingly muses that Luffy’s defeat here signals that he too isn’t fit to be ‘Joy Boy’ and so the mystery surrounding Joy Boy only deepens.
What’s significant here is that Kaido is the first person to mention Joy Boy without having read the Poneglyphs. A few chapters prior I suggested Kaido may also be one of a handful of individuals capable of hearing the Voice of All Things, joining Luffy, Roger, Oden, and Momonosuke. Perhaps his knowledge of Joy Boy comes from this ability.
Another possibility is that he learned of Joy Boy through Rocks D. Xebec. Of course Xebec himself is a character still shrouded in mystery who may have known more about the world and its history than the average pirate. Xebec wanted to be King of the World, and he dabbled in the world’s greatest taboos, according to Sengoku. And there’s no greater taboo for the World Government than the true history.
Anyway, now that we know Kaido knows about Joy Boy, we can assume he knows this too.
Therefore Kaido knows that there is a prophecy around Joy Boy liberating Wano and opening its borders or at least Joy Boy is connected to the Opening of Wano’s borders.
If we assume Kaido learned this information from his time in the Rocks Pirates, that means that Kaido came to Wano to subjugate it KNOWING Joy Boy would show up and the country would be freed from his subjugation.
Kaido basically put himself in Joy Boy’s way knowing his defeat was somehow inevitable if these prophecies are to be believed.
Does that mean that Kaido’s plan ALL ALONG was to DIE at Joy Boy’s hand?
*Theory by ImmaIvanoM