In One Piece Opening 22 you can see many things including manga spoilers mostly out of context, but at the very end is what can be considered a big spoiler which is Kaido’s Dragon Devil Fruit.
During this sequence you can see Luffy fighting him and, as he goes Gear 4, Kaido breathes fire of which Luffy emerges from with a red aura and hits Kaido before the opening ends.
My theory about this is that so far in the story Luffy has been working on improving his Haki.
In his fight with Katakuri the focus was improving his Observation Haki and as of the current chapters the focus is his Armament Haki as now Hyo taught him during the prison fight.
In chapter 923 when Luffy fights Kaido all of his hits seem to do nothing to him and rather than bulking up more which would be a real world solution, the solution here is to improve his Armament Haki. Now Luffy’s Gear 4 is done by coating his body in Armament Haki and inflating it. This can already mean it’ll get stronger since he’s improving his Armament Haki, but when I talked about Luffy emerging from the fire with the red aura in the Opening 22, this could be something else is added to it.
This could be the vulcanization of rubber which is done in one way by adding sulfur which is the third most abundant mineral in the human body to rubber and heating it thus making it harder. This could be the case as seen in the Opening 22 and wouldn’t be the first time One Piece uses rubber science as seen in Skypiea with Luffy’s resistance to lightning. The red aura surrounding Luffy as he comes out of the flames gives off the look as he’s been heated up by them instead of it not affecting him at all making me think this is the case even more.
*Theory by nijnaman
