In Kaido’s journey with Rocks D. Xebec, it’s possible that he stumbled upon some of Joy Boy’s story, as his captain was a member of the Will of D. As a result, it’s possible that Kaido knows the true identity of Joy Boy and is waiting for him to appear. Does this make Joy Boy immortal? Maybe, but it’s also possible that Joy Boy is a reincarnation.
This could explain why a young Gol D. Roger and Monkey D. Luffy look so much alike. It’s possible that Joy Boy isn’t one man, Joy Boy could be many men, reincarnated over the years until he finally becomes strong enough to take down the World Government. This could also be the meaning of the term “Will of D.”
A reincarnation that gets stronger with every passing member, sounds like an incredible opponent and it’s probably why Kaido is searching for him. We know that Kaido knows something about Joy Boy and that he expected Luffy to be him. How and why? Are things that we are yet to discover but we can at least speculate.