OK, we've had our fun, now it's time for an actual chapter prediction!
Title: Gear Fourth
Cover Story: Solo Journey of Jinbe, Knight of the Sea Vol. 28: "Jinbe and Wadatsumi set off, as the villagers cheer"
The streets of Dressrosa are in chaos as the citizens run away from the Birdcage, with the Marines accompanying them.
Maynard: You there! Move that rubble out of the way!
Citizen: Help us!!!
A man is stuck under a somewhat large pillar, and his scared wife is holding his hand, trying all that she can to free him. The Birdcage is getting closer and closer to them, only about a foot away...
Suddenly, the pillar flies off the ground, where it hits the Birdcage and is nicely minced.
The couple run toward the other citizens as Maynard reports back to Issho.
Maynard: Huh...we've moved all the citizens we could find to the foot of the palace.
Issho: Continue searching for other citizens. This Birdcage will surely still draw blood.
As Maynard runs off, Issho stares toward the top of the palace.
Issho: Damn you, Straw Hat...shall my bet fail yet again?
Meanwhile, a family is in their house. The Birdcage has reached the entrance, and now there is no way out. With no way to escape, all they can do is scream.
Cut to the palace roof. It's time now...
With a slip of the hand, Luffy brings out a small matchbox, and takes a bite out of it.
At first nothing seems to happen, but over time...black smoke begins to seep out of Luffy, and his body slowly becomes grey, though not as dark as a Busoshoku Haki covering, and is more splotched across his body. Occasionally smoke seeps out.
Doflamingo: What do we have here? That sure doesn't look pleasant, fufufufu!
Luffy: Huh...I was told never to do this...but I must save my nakama from you, Doffy!
Flash back to one year in the past. Luffy is on the jungle island, battling against a large lion. However, the lion is stronger than him, and continues to have the advantage.
Rayleigh: C'mon, Luffy! This lion may be one of the stronger animals, but you can do better!
Luffy: I know! I got this! Gomu Gomu no...Grizzly Magnum!
However, Luffy's Gear Third attack misses, and the lion hits him with its paw, sending Luffy flying into a cave.
Rayleigh: Sigh...he needs more control over his Third Gear...
The lion paces into the cave, ready to finish Luffy off. Luffy is struggling to control his body.
Luffy: Come on! I can't go into chibi form again!
The lion roars as it pounces toward Luffy. Unable to counter in time, Luffy is sent crashing into the wall of the cave, causing the minerals composing the wall to disintegrate into dust.
Luffy: Uggg...*koff* *KOFF KOFF KOFF*
Luffy blows the dust away as he struggles to get back up. But as he faces the lion, he realizes something is changing. His body is turning grey. And he feels...he feels invincible.
Some time later, Luffy wakes up on a mat on the forest floor. Rayleigh is tending to him.
Luffy: Urggg...what happened...?
Rayliegh: You don't remember? That doesn't surprise me...
Luffy: I...defeated it...
Rayleigh: Yep. He's lying here, in much worse condition.
Show the lion, beaten up and whimpering, sucking its thumb.
Luffy: How did I...
Rayleigh: The minerals in that cave reacted with your Devil Fruit. They made your body tougher...but listen to me, Luffy. You are never to go into that cave again. I was thankfully able to remove the effects of the mineral from your body, but it would otherwise permanently affected your body...and destroyed it!
Several hours later, Luffy is up and walking again. And of course, he returns to the cave. Luffy enters it and pulls out several boxes.
Luffy: I'll listen to you, Rayliegh...but I'll protect my friends no matter what...and if this is the only option, I'll take it!
Back to the present
Doflamingo: You certainly look fearsome...but how effective is it really? Athlete!
Doflamingo rushes at Luffy, aiming a string-imbued kick at him. This time, however, Luffy manages to block the overwhelming force with his arms.
Luffy then braces himself as he gears up for his punch. He throws his arm back, back...further back than it's ever gone before. Luffy trembles from the effort.
Luffy: GOMU GOMU NO...VULCAN PISTOL!
The increased force of the punch, coupled with Luffy's new natural durability, is powerful enough to send Doflamingo flying. Doflamingo lands on his feet, but he coughs up blood.
Doflamingo: You...that is not...ordinary rubber! But how long can you keep it up? Overheat!
Doflamingo whips Overheat at Luffy, which strikes him in the shoulder. However, Luffy is not sent flying, and grabs hold of the string.
Luffy: GET...HERE... DOFFY...ARRGG!!!
Battling against the extreme friction, Luffy manages to pull Doflamingo towards him. With a shout, he kicks Doflamingo in the midsection, causing the Shichibukai to stumble back.
Doflamingo: How...he's not even using Busoshoku Haki!
The jig is up, Straw Hat! Ever-Binding String!
Doflamingo whips five strings at Luffy, immobilizing the Straw Hat.
Doflamingo: Excess String Drive!
Doflamingo surrounds his foot with Haki imbued string and drives it deep, deep into Luffy's midsection.
Doflamingo: JUST... DIE NOW, STRAW HAT!!!
Luffy: I...will never die!
Overcoming his immobility, Luffy throws both of his arms back. They once again go far, far back, and Luffy wraps them around each other. This time, however, Luffy coats his arms in Busoshoku Haki. Doflamingo still has his foot buried in Luffy and cannot move effectively.
Doflamingo: You're still...?!
Luffy: GOMU GOMU NO...VULCAN RIFLE!!!!!
With incredible force, Luffy hits Doflamingo with both hands, sending the Shichibukai flying far, far away. Doflamingo crashes headfirst into the edge of the palace roof, creating a large amount of destruction. As the rubble falls and the dust clears, Luffy stands angrily, ready to beat Doflamingo to a pulp once and for all.