A/N: This is my speculation on the final battle between Luffy and Akainu. It also has my theory on who will be joining the Straw Hats.
Luffy gasped for breath, the battle having taken its toll. Who was he kidding? This whole war had been exhausting. Still, he wasn't about to rest. Not until he took this…monster down for pain he inflicted on him two years ago.
Across from him, Fleet Admiral Sakazuki stood bloodied. Clearly Luffy had giving him a tougher battle than even Whitebeard did. Still, he was not about to lose and let his sense of Absolute Justice be tarnished.
"Do you realize what attacking me will do to you, Straw Hat?" He sneered. "The entire Marine force will not rest until they kill you."
Luffy sneered back at him with disgust. "Well, I wouldn't have attacked you…if you hadn't murdered my brother!"
Sakazuki smirked. "Your brother had the misfortune of being born under a bad sign, same as you. It's my duty to uphold justice by extinguishing even the slightest evil."
In a rage, Luffy ran toward Sakazuki, his entire arm covered in Haki before delivering a powerful punch to his face.
"Justice?" Luffy mocked. "Justice?! You call destroying my friend Robin's home when all they were doing was harmless study of the past? Or murdering Ace just because he was the son of the Pirate King?! Or hunting me down all this time just because my father is the most wanted man in the world?! I may be a pirate, but I still know the sense between right and wrong. And from what I've seen…THERE IS NO SUCH THING AS JUSTICE!"
Enraged at Straw Hat mocking his sense of Absolute Justice, Sakazuki charged forward, his entire arm enveloped in magma. At the same time, Luffy charged at Sakazuki, his arm burning with Haki.
"Gum Gum…Red Hawk!"
The two fists clashed, resulting in an explosion that created a cloud over the area, leaving Luffy unable to see where his opponent was.
"Hound Blaze!" came a voice from the cloud.
Suddenly, a hound shaped burst of magma came flying at Luffy. Quickly, Luffy jumped to the side to dodge it.
"Great Eruption!"
Looking up, Luffy saw a gigantic fist coming down at him. Before he had time to strike back, the fist hit the ground, resulting in an explosion that sent him flying.
"Hell Hound!"
While Luffy was still in midair, a claw-like form of magma shot at him and struck him in the chest. His chest burning, Luffy crashed to the ground, weak from the onslaught he just received.
"It's all over, Straw Hat." Sakazuki said as he approached Luffy's down form with a magma fist. "And once I'm done with you, your crew is next. Not just them. Everyone you've ever corrupted. I will not rest until I've expunged every last trace of your evil from this world!"
Luffy gasped. His crew! Zoro, Nami, Usopp, Sanji, Chopper, Robin, Franky, Brook, Jinbe, Hajrudin, and Bartolomeo. Not just them. Every friend he had ever made since he had set sailed. He was going to kill them all!
NO!
He already lost his brother to this monster. He was not going to lose anyone else!
Sakazuki stood triumphantly over Luffy's prone body. At last, he would be able to finish what he started two years ago.
"Justice is served." He muttered as he raised his magma fist before bringing it down towards Luffy's back.
Before he made contact however, his arm was stopped by a hand that shot up and grabbed him.
Gasping, Sakazuki looked to see Luffy's hand had grabbed his arm without sitting up. Sakazuki tried to get his arm free, but Luffy had a tight grip on it. Just where was this brat getting all this strength from?
When Luffy finally sat up, he looked Sakazuki in the face with a look he never displayed before…pure hatred.
With incredible strength, Luffy hurled Sakazuki with his one arm into a nearby rock. Before Sakazuki could get up, Luffy clenched his fists as his arms were enveloped with Haki while Second Gear activated, causing both his arms to be enveloped in fire.
As Luffy looked to his opponent, he could still clearly see Ace's death as if it were yesterday.
"Gum Gum…"
Sakazuki driving his magma fist through Ace's back.
"…Red Hawk…"
Ace slowly dying in his arms before leaving him with a smile on his face.
"GATTLING!"
Before Sakazuki knew it, a barrage of fiery punches came flying at him. The attack drove him straight into the back of the rock, leaving his powerless to fight back, let alone defend. The attack was so brutal that Sakazuki nearly lost consciousness. But even then, Luffy didn't stop. Again and again he attacked for what seemed like hours with a ferocity he never used before.
With a final yell, Luffy unleashed a double punch which resulted in an explosion that obliterated the rock.
As the smoke settled, Luffy approached his downed opponent. When it finally cleared, there he was. The once mighty Fleet Admiral Sakazuki, his body bloodied and broken beyond repair. His life moments from going.
With a final bit of strength, Sakazuki raised his arm towards Luffy.
"This act…" he coughed, blood spewing out of his mouth. "Will only incentivize my men to hunt you down with greater force. Absolute Justice will not rest until all evil is extinguished!"
Luffy looked towards his fallen foe with disgust. "The only evil man I see…is the one I just killed."
Growling angrily, Sakazuki let out a final gasp of air before his head fell backwards, his arm falling lifelessly to the ground. Somewhere in the world, the Magma Magma fruit was regenerating.
Luffy sighed. At long last, his brother Ace had been avenged.
Weary, but still able to fight, Luffy made his way to find his crew. All that was left was to deal with the Blackbeard Pirates.