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.
Another idea is to create a backstory involving his family. Maybe he had a mentor figure in his family who inspired him to be strong, or there was some event in his family history that led him to pursue such extreme power.
You could show his early days of training in more detail. The struggles he faced, the times he wanted to give up but didn't. This would give more depth to his character and his seemingly effortless power now.
One way is to delve deeper into his training. Maybe add more details about the hardships he faced during his intense training regimen. For example, he could have had to overcome various natural disasters that occurred while he was training in isolation, like surviving a volcanic eruption near his training area.
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.
His hair could also be a plot device in a more comical way. Imagine a situation where his hair has a mind of its own and keeps getting him into embarrassing situations. Like it gets stuck in a door right before a big fight, and he has to struggle to free it while the villain is making fun of him.
Well, a lot of One Punch Man fanfictions about heroes seeing Saitama are really interesting. Some focus on their initial shock at his power. For example, Genos is always in awe of Saitama's strength in the original work, and in fanfictions, this can be explored even deeper, like how he tries to understand the source of that power.
Tornado of Terror would likely be one of the first to try and take on Evil Saitama. She's powerful and confident, but she would soon realize the extent of his power. Mumen Rider, despite being weak, would still stand up to Evil Saitama out of a sense of justice. He would be quickly defeated but his act of courage might inspire other heroes to step up.
One idea could be a fanfiction where Saitama's hair suddenly gains sentience. It starts to give Saitama advice on how to be more heroic in a sassy way. For example, it might tell him to change his plain hero costume to something more flashy to attract more fans.
The plot could involve Saitama teaching a group of new heroes how to face their fears. He would tell them about his own journey, starting as an ordinary guy who decided to become a hero just for fun. Saitama could teach them that no matter how strong the enemy is, one should not be afraid to take the first step. He might use his one - punch knockout strategy as a metaphor for facing difficulties head - on, even if it seems impossible at first.
One way is to introduce a personal conflict for Saitama. For example, he discovers that his extreme strength has unforeseen consequences for the world or for his relationships. This could lead him on a journey of self - discovery. He might have to make sacrifices or learn to control his power in a new way. This arc would show his growth as a character as he grapples with these new challenges.
I also like the scenes where Saitama is just casually hanging out with the heroes, and they're trying to figure him out. He doesn't fit into their idea of what a hero should be, and the awkward yet hilarious interactions that follow are a great part of these fanfictions. For instance, he might be eating a meal with them, and they're all secretly observing him, wondering how someone so ordinary - looking can be so strong.
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.