WARNING: The following contains spoilers for One Piece Chapter #1016 by Eiichiro Oda, Stephen Paul and Vanessa Satone, available in English now from Viz Media.
Chapter #1016 of One Piece revealed that the Straw Hats will be gaining a new member -- if he survives, that is. Yamato, the son of Kaido, formally announced his intent to sail along with Luffy and the others once Kaido and Big Mom have been driven out of Wano once and for all. Should he make it through the fight with his father, he will be the official tenth member of the Straw Hat crew.
After attacking Shinobu and Momonosuke, Kaido was confronted by Yamato, who claimed that he intended to sever their familial bonds for good in one final fight. Luffy had just been rescued by the Heart Pirates after being knocked into the ocean after his own battle with Kaido, but he intends to return for a rematch. Until then, Yamato plans on dueling the Beast Pirate Captain to keep him from going after anyone else.
Yamato and Kaido's relationship has always been destined for failure, with Kaido representing everything that Yamato's idol, Kozuki Oden, stood against. Not only was Kaido physically abusive to his son, but he also kept Yamato imprisoned on Onigashima since he was eight years old, even using explosive cuffs to make sure he didn't escape. While Kaido does respect Yamato's pronouns and identity, that doesn't make his actions any less horrid.
Originally, Yamato wanted to sail with Ace but wasn't able to, so he waited for Luffy to make his way to Onigashima to try and join his crew instead. The two have since bonded over their desire to take down Kaido, especially after Luffy freed Yamato of the explosive shackles, showing Yamato that Kaido really was willing to kill his own son to get his way. That, along with all of Kaido's other discretions, brought Yamato to where he is now.
The main question now in the One Piece manga is whether Yamato will survive his fight against his father. Yamato was able to go head to head with Luffy, even when the latter used Gear Third. He's been shown as a very capable fighter, so if he can hold on long enough for Luffy to recover and return to the fight, he might actually make it off Onigashima after 20 years.
However, Kaido has made it explicitly clear that he is willing to kill Yamato and isn't going to hold back against him, so Yamato needs to be ready to fight for his life and his freedom. Together, he and Luffy may finally be able to rid Wano of Kaido once and for all.
