Saitama's relationships would be completely upended. Friends like King would no longer be able to approach him casually. And his relationship with the citizens would change from protector to feared being. His new monstrous nature might also attract other villains and monsters, who could either try to follow him or challenge his new - found power.
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.
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.
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.
One way it could be imagined is if a new, extremely powerful villain emerged. This villain might have the ability to nullify Saitama's strength, perhaps by some sort of reality - warping power. In the fanfiction, Saitama could be caught off - guard by this new threat, and despite his best efforts, succumb to the villain's attacks. It would be a very dark turn for the One Punch Man universe.
In a romantic setting, their relationship might start off slowly. Saitama would probably be oblivious to Tatsumaki's initial feelings. Tatsumaki, being the proud and strong - willed character she is, would have a hard time expressing her emotions directly. It could start with small acts of kindness from Tatsumaki towards Saitama, like using her powers to fix his apartment when it gets destroyed in a fight. Saitama might just think it's a normal thing at first.
Genos would probably be even more in awe of Saitama. He already idolizes him, but with Saitama being S - Class, Genos might see him as even more untouchable. He could become more protective of Saitama's new status, trying to fend off any detractors or those who might try to take advantage of Saitama.
Deku would likely become more confident in his abilities. In the One Punch Man universe, he'd see Saitama's nonchalant attitude towards fighting and might start to adopt a similar mindset. He'd also have to adapt to the different types of villains. Since One Punch Man villains are often very strange and powerful in unique ways, Deku would need to be more creative with his strategies.
A cool plot might be a time - travel fanfiction. Saitama travels back in time to when he was just starting to become strong. He meets his past self and tries to guide him, but also has to deal with monsters from that era who are now different threats due to the time shift. And there could be a mystery element where Saitama has to figure out why time is acting strangely in the first place.
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.
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.