Disclaimer: Don't own One Piece or One Punch Man

Note: This is just a plot bunny that grabbed me and forced me to write it down.

One Punch King

Luffy was bored. He had succeeded, fulfilled his dream, but he still felt empty. It was all just too easy. There was no challenge throughout what was supposed to be a great adventure to find the One Piece and become Pirate King. He had wanted to become Pirate King so that he would be the freest person in the world, but he realised that the amount of power he had already guaranteed it. That's why the Pirate King is the freest. He had proved that he is the most powerful person on the seas, is acknowledged as such and therefore not challenged. The only thing that a Pirate King had that Luffy had been without was the title. Luffy realised that the lack of excitement came down to the fact that he ate the stupid devil fruit that Shanks was carrying around. When he had eaten it Shanks went as white as a ghost as he had been safeguarding that particular devil fruit for years making sure that it was never eaten. It was the most powerful devil fruit in existence: the one punch one punch fruit. It made whoever ate it a one punch man. With that title came invincibility, infinite strength and infinite speed. The issue was that as legend said it took away all possible thrill one could feel from combat. It wasn't a direct effect of the fruit, but when you know that you are going to win before a fight begins, the fight itself holds no excitement for you. The other issue was, as Luffy found out, the difficulty of keeping anyone you fought alive.

At first Luffy thought that it was a great gift, with that much power he would be able to protect anyone he cared for. That illusion started to fade when he first used that power to confront the bandits that made fun of Shanks. Having them explode under what should have just been a regular punch was sobering and showed Luffy that he needed to be extremely careful with his powers. The only upside was that Shanks and his crew didn't shy away from him, joking at how his punch was no longer like a pistol, that it was more like a cannon. The next time he really used his powers in a way that was noticed was when he destroyed Bluejam and his crew to protect his new friends Ace and Sabo. He felt bad for killing them, but to save Ace and Sabo it was worth it. Especially as they accepted him into their group after the event. The only issue that this event resulted in was the fact that Ace and Sabo wouldn't spar with him. That at least made sense to Luffy, he needed to learn how to control his strength before he could ever have a spar. He was sad when his brothers left to go on an adventure without him, but he would stick to the promise and wait to be old enough before chasing after them. Besides they would need that head start to ever stand a chance beating him.

When Luffy left on his own adventure he had three goals, find more nakama, find challenging enemies to fight and become Pirate King. He only succeeded in two of those goals. Captain Axe-Hand Morgan exploded, but that was fine he was a bad man and Luffy got Zorro's friendship out of the deal. Buggy the Clown deserved to be exploded for how much trouble he was causing the town he was in, and Nami started travelling with him after that event. Captain Kuro didn't deserve mercy after his betrayal of both his crew and Kaya. He didn't receive any and Luffy got Ussop's loyalty out of that clash. Don Krieg didn't put up a challenge, but during that adventure he actually saw someone he might actually want to fight. Dracule Hawkeye Mihawk, it was too bad that Zorro called dibs. He actually got some hope up when he went to fight Arlong. Apparently fishmen were stronger than humans, but a love tap later Arlong was sushi and Nami was truly a Nakama. Smoker was an interesting case for Luffy. He was someone with morals who fought for what he truly believed was right. Luffy didn't want to kill him and as such couldn't fight him. He didn't like running, but that was better than killing a good man.

The first time Luffy really got his hopes up was when he went to fight Crocodile. Apparently he was a Logia and was therefore immune to direct physical attack. Luffy was disappointed, when he punched the man it was apparently hard enough to turn his sand body into glass which was no longer under his control. This made it so when the glass shattered Crocodile died, or maybe he died when his chest stopped being sand and became glass. After that event Luffy decided to let his crew deal with as much as possible and only fight if they were in danger of dying. Bellamy was another opponent that Luffy decided not to kill when he met him. Luckily the man was reckless and jumped at people head first. Luffy knocked him out just by holding his fist in the way of the mans head rather than punching him. Enel was also a disappointment, but not near as much so as Crocodile he actually took a full on serious punch to hurt. This was because he had to punch hard enough to overcome the electrostatic repulsion of the electrons to force them to fuse and therefore no longer be electricity. By the time they had reached Water Seven the government should have realised what he was and not try to kidnap one of his Nakama, but that was what they did with Robin who had joined them after Crocodile's demise. It was especially stupid as Aokiji had only warned them about Robins past when they met, and made no hostile moves. It wasn't that hard to storm Enies Lobby. Luffy felt sorry for the marines he was forced to kill to make way for his Nakama, but it was their fault for stealing Robin. Lucci was supposedly a super powerful warrior, but to Luffy he was only good as a spy. Able to attempt an assassination and steal Robin without his knowledge. In a fight he was just a splatter of blood. After grabbing Robin and a new Nakama in Franky they set off again. They were waylaid by Moria, who was more of an annoyance than a challenge. He kept running away and using shadows to hide or to mislead Luffy. When Luffy eventually found him he went splat, but at least in the adventure they found a new Nakama in Brook. In the Sabaody Archipelago Luffy finally fought someone who took an ultra serious punch in Kizaru. The man didn't seem all that bad, but after exploding that World Noble the man seemed on taking out Luffy's crew. Luffy's regular and even his serious punch didn't affect the man as he was made of photons. However, his ultra serious punch created a black hole on the front of his fist which dragged in the photons and made them into something that was no longer light killing the man. Luffy then heard that Ace was imprisoned in Impel Down. He immediately stormed the place with his crew, killing Magellan on the way down. He freed Jimbei as well on Ace's insistence and the man joined his crew, they met Blackbeard on the way up, who was felled with extreme prejudice by Luffy as he was the one who captured Ace.

After freeing Ace everything just sort of fell into place. He allied with Whitebeard as a partner as the man was both happy at Luffy saving one of his sons, that Luffy was the brother of two of his sons (Ace and Sabo) and didn't want to fight someone who had ate the one punch one punch fruit. Whitebeard then offered Luffy's crew the training they would need to stand against the threats of the new world without relying completely on Luffy's prowess. The best thing Luffy learned in the training was conqueror's haki as it would allow him to avoid killing cannon fodder. At the end of the three years of training Luffy sent a message to Shanks asking for an alliance. Shanks replied that Luffy needed to defeat a Yonko first and then he would be fine with the arrangement. Luffy decided to go after Kaido. The fight was interesting as apparently Kaido's Zoan devil fruit turned him into a mythical beast that had the power of invincibility. That did add a bit of a challenge for Luffy as he needed to defeat the man indirectly, but it didn't take long for the one punch man to decide to just punch the Yonko into the sun rather than fighting him directly. With that Luffy ascended to the title of Yonko and Shanks allied with him. In the face of an alliance between the other three Yonko Big Mom decided to ally with them as well. This basically gave Luffy free reign over the seas. It only took him a year to find the One Piece after that, and the only reason it took that long was the distance he had to sail. He had been bored most of his life, and never truly found a good fight, but now he was Pirate King. Maybe he should start pursuing a relationship, at least that would probably be a bit of a challenge.

Note: Before I wrote this I was debating on making the fruit a human human fruit model: one punch man, but that wouldn't be as poignant because Luffy would be able to be a normal human allowing him to make fights a challenge. It would lose some of the parallel thought processes I tried to bring in from One Punch Man as well.