One way is to explore his life before he became a hero. Maybe he had a normal job and was constantly frustrated with the mundanity of it. This could have been the catalyst for his intense training.
A plot where Saitama travels to different dimensions in the One Punch Man universe. He could encounter strange versions of himself or other heroes and villains, leading to humorous and action - packed situations.
Identity exploration is also a theme. The sister might be struggling with being in Saitama's shadow or trying to find her own identity as a hero or just as an individual. She could be trying to make a name for herself without relying on her brother's fame.
One good fanfiction could be 'Saitama's Sister: The Hidden Power'. It explores the idea of Saitama having a sister with her own set of powers. In this story, she tries to keep up with her brother's reputation while also finding her own place in the hero world.
Saitama's true strength was recognized by many in One Punch Superman. With the help of his neighbor and teacher, Dr. Orochi, people gradually understood his strength. Dr. Orochi had once told Saitama that he had powers beyond that of a human, and that he could strengthen himself through training. In addition, Saitama's teacher, Hungry Wolf, also called him the " strongest human." In addition, other characters such as Superman and Atom also acknowledged Saitama's strength.
Saitama might be really frustrated with Mosquito Girl. Since he can defeat most powerful foes with one punch, but a mosquito girl would be a different kind of annoyance. She could be constantly buzzing around and trying to bite him, and Saitama would probably just try to swat her away like a normal mosquito at first. But when he realizes she's a 'villain' in her own right, he might just give her one quick punch to end the nuisance.
One possible plot could be that Tornado initially challenges Saitama out of pride. She sees his nonchalant attitude and wants to prove that her psychic powers are superior. Saitama, being Saitama, just goes along with it. As the battle unfolds, Tornado throws everything she has at Saitama, but he just dodges casually. Eventually, Saitama throws a light punch that knocks Tornado back, but not seriously hurt, which makes her realize his true strength.
To write a character - driven story about Saitama, focus on his boredom. Show how his extreme power has made his life lackluster. For example, you could write about his attempts to find a hobby that actually challenges him. Maybe he tries painting, but he's so good at it immediately that he gets bored. This exploration of his search for something to fill the void in his life can be a great character - driven story.
In a fanfiction, Mosquito Girl could develop a grudging respect for Saitama. After being defeated by him, she realizes that there are beings far more powerful than her. And Saitama, on the other hand, might start to see Mosquito Girl as more than just a villain. Maybe he discovers that she has some good qualities, like her tenacity. So their relationship could shift from pure enmity to a more complex dynamic of mutual understanding.