Kaido might talk about Joyboy again, it is about time we get some new revelations about the D. or the ancient history. or why Kaido himself decided to reside in Wano. who know Kaido himself might be a D., he knows about Joyboy and told Luffy that he could not be Joyboy either, i think he tried himself to be Joyboy too, but didn't succeed that is why he is depressive. being battle fanatic, and many of his antics might be come from his imitation of Joyboy feats, he wants to be him but he cannot.
It is just a speculation but like Oden he knew that Joyboy will go to Wano, and that is why he went there.
if that is the case he must be a D. since Joyboy for sure has to be a D. and it would be known in the prophecy too.
by the way the WG knows that too, that is why they are trying to keep Wano close at all cost.