They could have an adventure where they go on a quest to find a super - powerful, but very elusive monster. Child Emperor uses his gadgets and intelligence to track it down, while Saitama is just there for the ride, ready to punch it when they find it.
Well, it could be that Child Emperor creates some high - tech gadget specifically designed to analyze Saitama. He might think that Saitama is some sort of new type of hero that defies all known scientific principles. Saitama would likely go along with it, as long as there's no trouble and he can still get his grocery shopping done. Maybe they end up teaming up to fight a really tough monster that neither could handle alone, with Child Emperor providing the strategic planning and Saitama delivering the knockout punch.
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.
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.
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.
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.
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.
Well, one possible plot twist could be that Saitama's boredom turns into a malicious form. Since he's so powerful and always bored from easily defeating enemies, this new evil side might start creating stronger foes just to have a real challenge, but without caring about the destruction it causes. He could also turn against the other heroes, seeing them as weaklings not worthy of his time anymore.
In this fanfiction scenario of Saitama meeting Tornado, the story could be centered around their different ways of dealing with threats. Tornado is used to using her psychic powers in a more elaborate and showy way. Saitama, on the other hand, just punches. Their meeting could lead to Tornado being curious about Saitama's simple yet extremely effective method. Maybe she would try to get him to teach her his way, but Saitama would be like 'There's nothing to teach, I just punch'. This could lead to some humorous exchanges between them as they interact more.
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.