1) The Inflatable Castle
Imagine the fights! Luffy would be able to launch himself with a greater force, destabilize the opponents by rubberizing their footing or just wreck havoc on enemy ship by making everyone hop unstoppably. Quite a scary Inflatable Castle, if you’re a Devil Fruit user and you can’t find any footing eh?
Luffy becomes able to control the density of his own skin. Oh, this is quite serious, because now, he doesn’t even need Haki to deflect sharp things, he is as strong as the strongest tire developed by current civilization. Yes, I am talking about those tires on the Extraterrestrial surfaces, those tires that can withstand being in full-contact with lava.
Why am I calling this “New Engine”? Because Luffy calls his battle forms “Gears.” Why is this not a G5? Because now Luffy has a completely new “material” to create forms from. By changing the attributes of his skin, he would obtain a new body to train in. A new engine! And if it would be possible for him to shift Gears in this new body… He would shatter everything that stands before him!
Luffy shrinks into a Mini-Tontatta and one hits Kaido! Impossible right? Just hear me out.
Now, imagine he can do this on a whim, with all of his body parts. You think you are aiming for his head, but hey! Now he is smaller. You’re defending against his giant G3 attack, but his suddenly really tiny fist slips trough your guard, only to enlarge itself in a massive explosion attack behind it. Imagine a complete G3 Luffy giant, compressed to pipsqueek smaller than Tontatta.
This one is wierd and kinda scary, but in the same time really funny, because we’re talking about Luffy.
He becomes this insane thing which is nonstop contracting and retracting, seemingly liquid in movement but completely solid on the impact. WTF, right? WTF is this? Yes. That’s exactly how everyone would react, because if Luffy would decide to utilize this power on all of his body at once, he would look like a destabilized Meat Fire Hedgehog on acid. His body would not only emit smoke, but the tension in his body would cause him to fully emit flames. He could instantly attack from any part of his body. I don’t know, his belly button, creating a spear-like high pressure speed attack, while dodging everything with Observation Haki and while turning every impact into his own motion force.