@Jew D. Boy @Ravagerblade @Seth Hating aside, if Yamao joins, 1 thing I think will happen is: we will see Kaido in different light post Wano arc.
As things stand right now, Kaido is often viewed in bad light, for readers and for Yamao. Yamao hates her dad because he executed Oden. Straw Hats, 9 red scabbards and Wano villagers hate Kaido for what he had done to Wano country.
But there is good side of Kaido that Oda sometimes showed briefly:
- he executed the previous user of Mane Mane because she interfered his duel against Oden
- he slowly viewed to gain respect for Luffy. He doesn't fight Luffy solely to kill him like usual arc villains, but both of Luffy and Kaido are apparently enjoying their duel
- he adopted Ulti and Page One
- he saved King and King respects him much. In their flashback Kaido seemed to be not as bad person as he is now
- he probably saved and adopted Jack too
So if Yamao joins, in post Wano arc I think we will be showed the good side of Kaido, and Yamao will slowly change her view on her dad.
If theory about Pizarro being former Calamity is right, I think Yamao vs Pizarro can happen with this scenario: Pizarro trashtalked his former captain Kaido, comparing how shitty Kaido is compared to Blackbeard. EoS Yamao, or Blackbeard arc Yamao, who have heard and known about good sides of Kaido and changed her opinion of him, will get furious and fights him.
Then the former Calamity of Kaido will fall at Kaido's daughter's hand.