I'm quite sure there exists a 'Naruto is One Punch Man' fanfic. The world of fanfiction is vast and full of all kinds of creative crossovers. Writers might be intrigued by the idea of bringing Naruto's ninja skills and the unique world of One Punch Man together. For example, how would Naruto react to the overpowered villains in One Punch Man's world? And how would Saitama view Naruto's determination to protect his village? These are the kind of things that could be explored in such a fanfic.
Obviously, King himself is a very popular character in 'One Punch Man King' fanfiction. He is often the center of the story, whether it's about exploring his mysterious powers or his relationships with other heroes. Another popular character is Saitama. Since they are both part of the hero association, there are many fanfictions that involve their interactions, like Saitama being curious about King's supposed feats of strength.
One interesting 'One Punch Man King' fanfiction could be where King actually has some hidden powers that he's been suppressing. In this fanfic, he might start to slowly reveal his true strength during a really tough battle against a new and extremely powerful monster. It could be a story that focuses on his internal struggle between wanting to keep his low - key image and having to step up to protect the city.
One possible storyline could be Harry arriving in the One Punch Man world and being mistaken for a new hero. He then has to prove himself while also trying to figure out how to get back to his own world.
One possible plot could be that Naruto is somehow transported to the One Punch Man universe. He might initially be confused by the different world but then start to adapt. He could team up with Saitama to fight against some powerful monsters. Along the way, Naruto tries to understand Saitama's seemingly effortless power while also showing Saitama the value of his own ninja way.
One common theme is power comparison. Since both Naruto and One Punch Man have characters with extraordinary abilities, fanfics often explore how Naruto's chakra - based powers stack up against Saitama's strength. Another theme is the clash of cultures. Naruto's ninja world has its own set of values and rules, which is different from the modern - day - ish world of One Punch Man.
The villains in One Punch Man might also have interesting reactions. Boros, if he could watch a fanfic, might be confused at first. He's used to his own view of the world and his place in it. But then he might see the fanfic as a chance to understand his enemies better. He could also be a bit angry if the fanfic makes him look weak or stupid. Overall, it really depends on the character's personality and their relationship with the original One Punch Man story.
The villains would have diverse reactions too. Garou might see the fanfic as a source of inspiration for his own 'evil' plans. He might think that some of the ideas in the fanfic could make him an even more formidable foe. However, if the fanfic makes him look like a complete fool, he'd be enraged. Deep Sea King would be confused by the fanfic, not really understanding the concepts that are different from his simple view of the world. But he might also be intrigued by the new versions of himself presented in the fanfic.
Yes, there likely is. The Naruto and One Punch Man fandoms are both quite large, and it's common for fans to create crossover fanfictions. These crossovers can be really interesting as they combine the unique elements of both universes, like Naruto's ninja skills with Saitama's one - punch power.