ONE PIECE GOLDGear second was blood, gear third was bone, my idea is gear fourth is muscle/flesh.
I thought it’d be interesting if he took full control of the elasticity of his body. In my idea of Gear Fourth, he retains the same shape, but gets slightly darker, like a lighter version of armament haki.
He’d fight in this way without stretching, as we’re used to with the rest of his gears. When he fights in Gear Fourth, he takes any blow, or counters it with one of his own, has his body internally vibrate, strengthening the attack, and returning the attack with his own power PLUS the force he was hit with almost immediately.
Say someone punches him, and Luffy counters his punch by punching the dude in the fist (because that’s how he fights) the dude’s strength shockwave travels through Luffy’s body into his attack on the other side of his body. When Luffy swings and hits the dude with his next punch, the force of that punch is Luffy’s PLUS the force the dude hit him with at first.
Now, he could chain these attacks, as each successive attack could be considered stronger than the last (force waves bouncing back and forth inside of Luffy’s body, allowing Luffy to strike at the right times to build them up and make them stronger).
Another idea I thought of is Luffy could go Gear Fourth, and start punching into his hand as he usually does before battle, and he just keeps punching harder and harder, faster and faster, building up the force, and then smashes the big bad in the face. That’d be an interesting introduction, we just see Luffy one shotting a big bad he’s had trouble fighting previously with Gear Second and Third.
The downside, because every awesome bonus must have a downside is two fold:
One, if he doesn’t connect with a hit after building up sucessive hits, the force just propagates uncontrollably throughout his body and he goes back to regular Luffy, with his limbs/body flailing everywhere as the forces he’s been bouncing back and forth in his body are unleashed. Doesn’t nessecarily cause bodily harm, but it makes Luffy temporarily lose control, for longer and stronger amounts of time based on how much force he’s accumulated.
Two, and the real issue, if Luffy continues to attack like this and doesn’t win in time, the forces inside of his body could build up enough to literally tear him apart. With this, if he gets to a certain point where he hasn’t won yet, he’d have to force himself to miss and hope that the guy he’s attacking doesn’t take advantage of the fact that he’ll be vulnerable when he loses control of the forces inside his body.
Maybe after some training he can learn to ground himself before building up too much power.
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