Here is my theory of Luffy’s new Gear 4 form that he will use to fight Kaido.
First, why do I think it will have an inspiration of King Kong?
Well, that is because I believe there is a Kaiju theme among the Beasts Pirates.
Kaiju is a Japanese film genre that features giant monsters, usually attacking major cities and engaging the military and other monsters in battle. Godzilla is an example of a kaiju.
Kaido’s Disaster King the Conflagration ate the Ryu Ryu no Mi, Model: Pteranodon, an Ancient Zoan-type Devil Fruit that allows him to transform into a pteranodon, making him similiar to one of the most recognizable monsters in the Godzilla verse, Rodan.
Rodan’s japanese name is Radon, “The Japanese name Radon is a contraction of Pteranodon.”
It seems like too much of a coincidence for King not be inspired by Rodan, with both being Pteranodons.
With King being second in authority among the Beasts Pirates it makes sense that one of his inspirations comes from such a powerful monster as Rodan, (other inspirations for him could be plague doctor in hybrid form and the classic devil with fire and wings).
So now when I have established a most probable link to Kaijus, it is time go on for Luffy and King Kong. King Kong is considered a Kaiju in Japan.
Luffy’s Gear 4 forms so far, Boundman, Tankman and Snakeman have something in common.
Both Tankman and Snakeman are an upgrade to two of boundmans abilities, as you can see here:
Luffy also got the attacks, Gomu Gomu Kong Gun and Gomu Gomu King Kong Gun, and if Luffy’s new form, let’s call it King Kong Man, will be based on his Kong attacks, then just like his other Gear 4 forms, this King Kong form should enchant his Kong attacks by a lot.
If Luffy use this form to fight Kaido, it will be like a King Kong vs a gigant reptile-like monster.. oh that sounds familiar, there is actually a movie about it.
King Kong Vs. Godzilla, a japanese movie from 1962.
And as you may know, Oda gain a lot of inspiration from movies, especially old classics.
*Theory by Revilo D Oliver
