Honestly I don’t know what Rox flashback could do to his character to save Kaido.
Kaido was one of if not the most hyped villain in one piece, but beyond his hype he’s lackluster.
- His ideal of bringing those at the top down to prove themself at war was awesome. And if oda focused on this he would’ve been a great villain.
- Oden killed Kaido. Everything cool about him was just turned into Oden wank which made him much more lame
- Beyond that, the forced “respect” between him and luffy was so bad for both characters.
Big mom got power disrespected. However, Kaido got character disrespected.
I believe Kaido is one of One Piece's greatest villains, but hasn't had his character arc finished yet.
Kaido had no dreams at all growing up. He grew up as a warrior, and when the Vodka Kingdom sold him out, he had nothing.
Rocks gave Kaido the dream that the strongest person in the world could actually change it. But, then, Kaido saw Rocks fall, and realized just power wasn't enough, since his Captain was the strongest pirate, but still lost.
So, what did Kaido turn to? Joy Boy. I think Rocks could have put this in his head, too, telling Kaido he could change the world. When Kaido talked to King while saving him, he seemed so happy and full of life. I really believe Kaido thought he could be Joy Boy, then realized he wasn't. "I'm the only person that can change it!"
And, then, he just fell apart. If Kaido couldn't be Joy Boy, he would create Joy Boy. "I'm here because it's Wano, Yamato!" Kaido KNEW Wano would produce Joy Boy, the man who would defeat the WG, so, he set Wano up to both lure in Joy Boy, and give him an army to wield later.
The starvation, slavery, oppression, weaponry, all of it goes against what Joy Boy stands for; Kaido knew that, and kept trying to lure Pirates to Wano, to find the real Joy Boy. If he couldn't BE Joy Boy, he would make him.
But, at the same time, what has Kaido always talked about? A giant war. Kaido KNOWS from Rocks that a giant war is inevitable. His weapon stockpiling, creation of a giant army, hunt for the Ancient Weapons, are because Kaido knows the war Doflamingo talks about is going to come.
Kaido set himself up to be the last obstacle to Joy Boy, so that, once Joy Boy defeated him, he'd have a giant army full of weapons and Devil Fruits to command.
This isn't even getting into Kaido's connections to the Ancient Oni race.
Kaido is a great character, Oda just hasn't dug deep into what makes him the way he is, yet.
