In a fanfiction, Saitama could sacrifice himself to save the world. There could be an all - powerful, otherworldly threat that is about to destroy the entire planet. Saitama realizes that the only way to stop it is to use all of his power at once, which he knows will kill him. So, he makes the ultimate sacrifice, and in a blinding flash of power, he defeats the threat but dies in the process.
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.
Saitama would be shocked at first. He's used to Genos being around, always eager to learn from him. He might stand there in disbelief for a while.
The other heroes in the Hero Association would be in shock. Saitama was like an unbeatable force among them. His death would make them realize how vulnerable they really are. They would have to reevaluate their own strength and the safety of the world. Some might be inspired to train harder, while others could be paralyzed by fear.
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.
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.
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.
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.
The story could start in the middle of a fight. The character, now reborn as Saitama, is in the middle of a battle against a monster. They instinctively use Saitama's one - punch power to defeat the monster and then are left standing there, realizing what has just happened. They then start to think about how they got there and what they should do next in this new life as the world's strongest hero.
Genos would be an obvious choice as his student. Genos is always eager to learn from Saitama and improve his skills. Also, some of the lower - ranked heroes like Mumen Rider could be his students. Mumen Rider has a strong sense of justice but lacks in power, and Saitama could teach him some basic self - defense techniques.
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.