In the latest chapter Kaido mentioned that he has a son.
Kaido’s son is most likely a new character, if I had to guess, and he has probably been written as a reflection of Momosuke.
Wano arc is built trying to create a clear symmetry between the two sides of the conflict: each side has a traitor, an alliance and, with this last chapter, an heir…
Maybe this can be set as a surprisingly emotional weakness of Kaido, who defeated Oden by abusing of his paternal instincts. Maybe his downfall will come the same way? Or maybe we will see how heartless Kaido truly is when he sacrifices his own son for a chance of victory?
Another option is that he will serve to contrast to Momosuke. Momonosuke’s arc is about trying to be worthy of his father shoes, although he is not born with the same superhuman strength as Oden’s. Maybe Kaido’s son is pretty strong, but, for any reason, he isn’t worthy of inheriting his leadership role.
We should remember that one of the main themes of this arc is loyalty: Kaido getting it by overpowering his followers and Luffy getting by inspiring his followers, Eustass Kid being betrayed by his alliance, Law’s crew being accused of betraying his allies and the two double agents shared by Orochi and the Scabbards.
I can see Kaido’s son working to develop this theme: When someone should inherit someone loyalty? Is when he is as strong as his father? Is simply by sharing his father blood? Or maybe is there something else to it…?