Interesting development.
We already suspected by the manner Kaido spoke about his son that he kinda fell out of grace somehow. But to have him help Luffy directly is pretty shocking.
It's also a bit unlike Oda to have this from the get go, normally he sets it up and then comes with the twist. Now our first introduction to Yamato is the rebellious son.
He asks Luffy for his identity so it's not someone Luffy met before, yet he has strong motivation to help him out.
Maybe some old Wano connection or just because of deep hatred against his father. Wonder where this will go.
I also like that Luffy, Zoro, Sanji are still having to put up a good fight to go against headliners and the flying six. Same thing we saw in Whole Cake Island where the Big Mom pirates were shown as no pushovers. In a lot of other manga the protagonist would rip through these people and go straight for Kaido. Luckily One Piece has a pretty good grip on the power creep.
It just goes to show much of the original yonkou are just monsterous human beings