A plot could involve a villain who has the power to control hair and targets Saitama because his hair is so unique. This leads to a battle where Saitama has to figure out how to fight without using his hair as a weapon, which is quite difficult since his every move usually involves his head (and thus his hair) in some way.
One idea could be that Saitama's hair gives him some new, unexpected power. For example, his hair could start to glow and act as a power amplifier, allowing him to defeat enemies even more easily than before. This would be a fun twist on his already overpowered character.
The plot could revolve around a villain who targets Saitama's hair specifically. This villain might believe that Saitama's power is directly related to his hair and tries to steal it or cut it off. Saitama then has to protect his hair while also dealing with the normal chaos of fighting monsters in the city.
One idea could be Issei getting transported to the One Punch Man universe and trying to prove his worth among the powerful heroes there. He might start by challenging some of the lower - ranked heroes and learning the ropes of this new world.
Another plot could involve Sii being a civilian who accidentally gets caught in the middle of a fight between Saitama and a villain. Sii then discovers they have a latent psychic ability that they use to help Saitama in the fight, and this leads to Sii becoming an apprentice of sorts to Saitama, learning about being a hero.
In a One Punch Man fanfiction, a great plot could be the origin story of some of the minor heroes. For example, how Mumen Rider got his bike and started his hero journey. Another plot could be a tournament among the heroes to see who is the most skilled (not the strongest since that's Saitama). And a story where Saitama has to deal with a PR disaster for the Hero Association because of his non - traditional hero behavior.
In a One Punch Man Blizzard fanfiction, a great plot could be that Blizzard is challenged by a rival group who claim to be stronger. Saitama, in his boredom, decides to help Blizzard prove them wrong. He does this in the most nonchalant way possible, like just one - punching all the opponents while eating an ice - cream. This makes Blizzard both frustrated and in awe of him. Another idea is that Blizzard discovers a hidden power within herself after being inspired by Saitama's confidence in his own strength.
One idea could be an average guy suddenly gaining a power similar to Saitama's but in a very limited way. He tries to use it to help in small ways around his neighborhood, like stopping petty thieves or helping old ladies cross the street. But he constantly struggles with not being able to fully control it, leading to some comical situations.
Another plot could be that an S - Class hero loses their powers. They then have to start from scratch and learn what it's like to be a normal person. Along the way, they discover new aspects of themselves and the world around them. This could lead to them developing a different kind of strength, not just physical but also emotional and mental, which in turn could help them regain their powers in a more evolved way.
One idea could be that Naruto somehow gets transported to the One Punch Man universe. He teams up with Saitama to fight a common enemy that has the ability to absorb chakra and is also super strong physically. They start off not understanding each other's powers but gradually learn to work together.
In a fanfiction, we could see Sweet Mask being secretly a huge fan of Saitama. He could try to get closer to Saitama to learn his secrets of being so powerful. However, Saitama is completely oblivious to Sweet Mask's admiration. This could lead to some comical and interesting interactions between the two characters.