I think Kaido’s flashback will start in the next episode (987). He saw Oden in the last chapter (986) in a mini flashback before the Nine Scabbards landed their attacks.
Furthermore he is falling down, which is in general a very good way to start a flashback and generates a good transition to what his flashback is going to be about.
Before I start with how I think the flashback will be, I have to explain Kaido’s character a bit. Kaido seems to be the typical strong, manly pirate, who is obsessed with creating a powerful army to conquer the world. He would even go so far as to ally with his former crew mate and current rival Big Mom in order to achieve his goal of more power (aka one piece, ancient weapons, pirate king).
Contradicting his actual character shown in Onigashima he was firstly introduced as suicidal, someone who wants to kill himself and already tried all different ways of killing himself, going as far as to try to jump off Sky Island to do so.
I think he wants to kill himself to die an honorable death such as Oden in order to get Yamato’s affection. Just think about it, Kaido respects Oden due to him being the first one to injure him and Yamato sees himself as Oden. Kaido is somewhat fascinated by Oden but on the other hand sad, because he can’t get Yamato to see him as a role model. That’s why Kaido is trying to kill himself only (!!!) when he is drunk in order to gain Yamato’s affection.
Falling down after the attack of the samruai at the end of chapter 986 would be a great oppurtunity to start this flashback because it resembles his introduction, where he wanted to commit suicide. With him dying facing the Nine Scabbards Kaido could settle things mentally for himself (surrounding Oden) and reclaim his status as role model in front of Yamato if executed correctly from Kaido.
The problem is he just can’t die because he is a monster and is trying to act according to it both emotionally and physically while being sober. Only when he is drunk we can see his true character and how he is affected by Yamato not loving him. He wants Yamato to lead the Beasts Pirates and be Shogun which shows us that he holds a certain “love” for Yamato. Furthermore this could also explain why Kaido wanted to come to Marineford: in order to save Yamato’s friend Ace and maybe dying an honorable death while doing so which would result in gaining Yamato’s affection.
You all know Oda loves to write antagonists in a way where we can sympathize with them in the end. That’s why I think a flasback of his relationship to Yamato is very fitting, because it would also give Kaido a deeper character instead of just “I’m strong and want to be powerful”.
*Theory by batuprezi
