Let’s first go over what we have so far
G2 uses the Circulatory System. Luffy pumps more blood through his body supplying it with more materials/second which makes his body quicker and stronger with am emphasis on speed. Because he is using supplies from inside his body quicker he will be drained of energy quicker. Luffy will only state in this state when he needs to so that he can save energy.Luffy got the idea from watching the CP9 fight using rokushiki. Luffy seems to have based it off of Soru which allows the user to move at high speeds.
Luffy got the idea from watching the Elbaf Giants fight. He saw how big they were and how much power they had because of it.
G4 uses the Muscular System. Luffy coats part of his body in busoshoko kai, then creates an opening in his body to pump in air. This time he inflates his muscles. This, like G2, increases Luffy’s speed and strength with a emphasis on strength (Luffy still uses a type of soru in this form for speed). This also gives Luffy more control over his muscles allowing him to change direction by stretching a different part of his muscles mid-attack. Because he is coated with busoshoko haki, Luffy is more durable, but he maintains his rubbery protection from blunt attacks as well. To make his attacks even stronger, Luffy uses compression. By building up tension within himself he is able to release more force. This makes his attacks a lot stronger and allows him to “fly” like he is using geppo. However, there is also a time limit to G4 similar to G2. We are not yet sure why.
Luffy’s inspiration for G4 came from seeing how powerful big muscles were on his Nightmare Luffy form while fighting at Thriller Bark and watching the Cp9 “walk” on air with geppo.
Ok. So now that we understand Luffy’s previous gears, let’s discuss what he uses as the foundation of them. The body’s systems.
The first three systems on this diagram allow Luffy to use his current gears.
G2 Circulatory System
G3 Skeletal System
G4 Muscular System
Other bodily systems which will most likely not be used are the Lymphatic (relates to circulatory system and deals with toxins) Excretory, and Reproductive systems.
That leaves us with the possibilities of the Nervous, Respiratory, Endocrine, and Digestive systems.
Nervous System – the body’s control system. By stretching this system more of Luffy’s body will be covered by nerves making it so he can have complete control over his body similar to how he has better control while in G4. However, if he could sense his own body better by stretching the nervous system then he could perform, with the help of kenbunshoku haki, an ultimate form of the Cp9 Kami-e (dodge). He could also control his muscles better to use the CP9 Tekkai (hardening) and perhaps combine this gear with others to improve control over them.
Respiratory System – the body’s air system. Though I do believe Luffy already uses this, I’m sure his Lungs normally expand extra when he needs to use G3 and G4, there is a chance that Luffy could expand the respiratory system to its capacity to pump absolutely absurd amounts of air into his body allowing him to achieve the ultimate combination of G3 and G4 and become a giant!
Endocrine System – the body’s hormonal system. If Luffy stretches out the size of this system he could perhaps use this gear to produce more hormones quicker. He could have been inspired by seeing Ivankov’s techniques and use this gear to give himself a shot of adrenaline when getting tired from the strain of G2, G4. He could also produce more red blood cells with erythropoietin which could make his G2 way stronger. I also think this one is stretching it since Luffy is not too bright and reading through different hormones confused even myself, but who knows? Maybe Ivankov can explain something to him later on… If you want to check out some cool hormones check this page. Though it will probably hint more of Ivankov’s powers than Luffy’s.
Finally, the one that I wouldn’t put past Luffy at all to use, the
Digestive System – the body’s food system. Luffy loves food and he can eat a whole lot, but if he stretches his digestive system he could probably eat an infinite amount. I can imagine Luffy seeing a snake expand as it eats it’s prey when he trained on Amazon Lily and thinking that is something that is sugoi XD. He could have also gotten the idea from Wappol. Anyway…. this would allow Luffy to have an enormous amount of energy going into battles and it would allow for Luffy to regain energy a lot quicker as well. Though this could again be something that he already uses to some extent when eating normally.
Based on knaal’s idea we also have:
The Integumentary System which protects the body.
This system with the help from some of the above systems (mainly the Nervous) could become a powerful gear which could be used as an ultimate defense mechanism. By strengthening the skin with air and stretching the nervous system to give better control and use of tekkai, Luffy could become a defensive monster.
From these systems I think that the Nervous/Integumentary and Respiratory systems are the most likely to become the next gears, respiratory being the most likely IMO since he uses his lungs for two other gears already and loves giants.
I also do not believe Luffy will use other Cp9 abilities that I did not mension like the Shigan or Rankyaku since they go against his fighting style.
I will also mension that I believe G4 to have more forms, one of which I think will be a super compressed Luffy based on the “down side” of G3. We have to remember chibi Luffy is another possibility just like Nightmare Luffy became a true form with Gear 4th.
*Theory by Cpt.