Another plot could involve Saitama using his new S - Class status to reform the hero system. He might see all the flaws in it, like how some heroes are more concerned with fame than actually saving people. So, he starts making changes from the inside. He could introduce new training methods or even a new ranking system that focuses more on a hero's selfless deeds rather than just their power level.
If Saitama became a monster in fanfiction, his physical appearance would surely change drastically. His bald head, which is now a symbol of his simplicity and power, might take on a more menacing look. His eyes could turn red, and his muscles might bulge in an even more grotesque way. This transformation could also affect his relationships. Genos, for example, would be in a difficult position. He idolizes Saitama as his master, but if Saitama becomes a monster, Genos might have to fight him to protect the city.
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.
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 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.
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.
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.
First, understand the characters well. Saitama is bored with his power and just wants a good fight, while Boros is a proud and powerful alien seeking a worthy opponent. Then, come up with a unique plot. Don't just rehash their fight from the original. Maybe create a new world or situation for them. For example, a world where all heroes have lost their powers and Saitama and Boros have to work together to save the day. Also, good dialogue is key. Make it sound like how the characters would actually talk. Saitama's simple and straightforward speech and Boros' more grandiose way of speaking.
One common theme is the exploration of Saitama's power in different universes. For example, when he crosses over to a universe with magic, how his physical strength compares to magical abilities.