Saitama's origin story is quite interesting. He was an ordinary man who was frustrated with his job - hunting and general lack of success in life. He decided to become a hero just for fun. He started a very intense training regime that included 100 push - ups, 100 sit - ups, 100 squats and a 10 - kilometer run every day. Through this extreme and consistent training, he gradually became so powerful that he could defeat any opponent with a single punch.
Saitama is a very powerful hero in the One Punch Man universe. He can defeat any opponent with just one punch. He has a simple appearance and a somewhat laid - back personality.
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.
Saitama's rank in the web comic is sort of unconventional. His strength is off the charts, making it difficult to slot him into a specific rank. You could say he's in a league of his own, far above the typical rankings.
Sometimes King is paired with Saitama. King, known for his false reputation of being a super - strong gamer in the original story, could be in interesting interactions with Saitama in the fanfiction's gaming context. Maybe they compete in a gaming tournament within the fanfiction, and Saitama's effortless wins baffle King.
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.
Genos could be an unexpected opponent. He might be forced to fight Saitama due to some mind - control or a misunderstanding in the rules of the tournament. It would be really interesting to see how Saitama would handle fighting his own disciple, and how Genos would fare against his master's incredible strength.
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.