- King got Kaido's dragon. Queen got Kaido's Oni (Ice Oni). Jack got Kaido's Ls. It's a nice gesture of the huge respect and belief they have for their captain that they try to encapsulate his image in their deadliest attacks.
- King being a magma user, at least for one attack, shows where he stands. Oda portrayed magma as superior to fire with Akainu Vs Ace, differentiating King among the many fire users from Holdem to Kin'emon and putting him one step above. Sanji's flames unleashing bolts of lightning is the same principle. It also marks progress for Zoro, growing from cutting fire against Kaido to cutting King's magma.
- Sanji rescuing Chuji and saving Osome reminds us of where he came from. He was a failed child prisoner in the Germa kingdom, couldn't save his mice friends and protect women like Cosette. Powerless with no control over his destiny. Our boy has come a very long way.
- Honestly, King being revealed as a test subject is a low key big reveal. Someone like Judge could have swiped some DNA for his research, though Sanji's Lunarian connection remains iffy with King's regular eyebrows/dark skin/white hair. We know Kaido's DNA resulted in Vegapunk's artificial fruit. What did the World Government create out of Lunarian DNA?
This is a good callback to Arlong Park.
Let's not forget Kuina was using her biology as an excuse. Zoro is definitely sick of hearing this line.
I love that the villain who constanly monologues about pirates betraying each other started out by telling his highly sought after Lunarian first mate that he's never going to sell him out.
Who broke post-Rocks Kaido? Because this is Wano arc, I'm going to guess it was Oden. Shanks was too young. We know his relationship to Big Mom. Roger and Whitebeard avoided Kaido. Kaido never flashed back to anyone else when Luffy hit him.
Oda shows us Kaido's physical transformation here. He stopped dressing like a rebellious punk after Oden died. His invincible body bears a big mark of shame. Kaido saw that Oden was undone by his trust in Kanjuro and Orochi. That Oden became soft, willing to trade power to bargain for the lives of Wano citizens. Kaido couldn't even bring himself to kill Oden's kid because of how bad he felt for the foolish lord and how much he saw himself in him, with his Yamato the same age as Momo. Kaido also began to see death as a form of completeness for how Roger and Oden achieved eternal glory with theirs.
Thinking he learned from Oden's best and worst, turtling up for 20 years in Wano warped Kaido into a dark, somber creature obsessed with power and death. He wasn't going to get betrayed. Strength and fear is how he maintains control, breaking his subordinate's wills and replacing it with his own. Oni subjugating humans is the natural order, as he tells Yamato. Treating his crew like disposable experiments, reflecting the Beast Pirate's origins in Punk Hazard, is favorable on the path to Pirate King. On the path to change the world like he initially desired.
King of Hell, Three Dragon's Style: Dragon's Hell/Damnation. The King(Ou) from Conqueror's Haki. King of Hell to compliment Enma, king of the Underworld. And a three sword style upgraded to three dragon's style. It all sounds so badass, but then 136 Buddhist hells get tweaked to 103 mercies...because Oda is a funny guy.
With the official scan's quality it looks like King uses "Extra Large Imperial Flaming Wings" (Karyuudon = Curry Udon) with no fire on his back, leaving him exposed so we never actually got to see Zoro surpass the fire defense. A shame, but maybe Zoro cutting diamond is being saved for a future moment. And it does add to the appeal of this fight that Zoro doesn't simply win by cutting harder, but rather smarter (technically he did level up to slicing magma). With King experimented on and serving as an intro to Lunarians, their story to be fleshed out in a later arc, the uncuttable fire defense could make a return.