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Gear 2 augments Luffy’s blood to increase base speed.
Gear 3 augments Luffy’s bones to increase mass and attack power.
Gear 4 augments Luffy’s muscles to increase base speed, attack power (Bound Man), defense (Tank Man) and attack speed (Snake Man)
My theory is that Luffy, inspired by seeing Kaido in his dragon form, will develop a Gear 5 that will allow him to reshape his skin into scales and harden them with the advanced form of Armament Haki exhibited by Katakuri.
These scales will allow his body to stretch while maintaining an impenetrable defense. Adding the blood flow of Gear 2 to the mix could interact with Armament Haki at high speeds like a full body Red Hawk, setting his entire body on fire.
The only thing I wonder with this theory is how he’d maintain the insane boost in speed and strength offered by Gear 4. Perhaps Luffy will learn to develop these normally, as any Non-Devil Fruit user would have to do anyway.
*Theory by cntoruga