In " One-punch Superman," the Weirdo King of the Weirdo Association really appeared. He was a very powerful freak with all kinds of superpowers, including invincible strength and speed, and the ability to transform at will. Together with the other monsters, he threatened the safety of the city and was eventually defeated by Saitama.
One interesting storyline could be about a new hero rising through the ranks of the Hero Association in an unexpected way. Maybe a hero with no super strength but extremely high intelligence devises unique strategies to defeat monsters, which challenges the traditional view of what a hero should be in the One Punch Man universe.
In a battle between One Punch Man and Superman, it's really tough to say. One Punch Man can defeat any opponent with just one punch. But Superman has incredible strength, speed, and a whole bunch of other powers like heat vision and flight. If One Punch Man managed to land his one punch, he could win. However, Superman's speed might make it difficult for One Punch Man to even get a hit in. So, it could go either way depending on the circumstances.
Often, original characters who are strong but also have a complex backstory are popular. For example, a character who was once a villain but turned to the side of the heroes for a very personal reason. Their struggle between their past and their new identity as a hero can be really engaging.
Similarities? Well, both are super - powerful. They both are often the ones to save the day in their respective universes. Differences? One Punch Man's power is very straightforward - that one punch. Superman has a whole range of powers. In fanfiction, One Punch Man might be more of a comical character in his overpoweredness, while Superman is more of a traditional hero figure.
If Saitama became a monster in fanfiction, his physical appearance would surely change drastically. His bald head, which is now a symbol of his simplicity and power, might take on a more menacing look. His eyes could turn red, and his muscles might bulge in an even more grotesque way. This transformation could also affect his relationships. Genos, for example, would be in a difficult position. He idolizes Saitama as his master, but if Saitama becomes a monster, Genos might have to fight him to protect the city.
Another storyline could be that Thanos, aware of One Punch Man's power, tries to use the Infinity Stones to either eliminate him or make him an ally. There could be a long - drawn - out battle where One Punch Man has to constantly dodge Thanos' attacks while trying to find a way to land his one punch. In the end, he manages to do it and saves the day.
For a One Punch Man Monster OC in fanfiction, consider its weaknesses. Even though it's a monster, it can't be all - powerful. Maybe it has a fear of bright lights, so heroes could use that against it. Its personality could be really cocky because of its unique powers, but that could also be its downfall. It might underestimate its opponents and get beaten as a result.
It's likely about King from One Punch Man in a visual novel format. It might focus on his adventures, his relationship with other characters in the One Punch Man universe, and perhaps explore his unique 'pretend' hero status in more detail.
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.