A fanfiction could explore what would happen if Saitama was transported to the world of Youjo Senki during a war. He might be completely unfazed by the chaos around him at first, but then gradually get involved in the fight against the enemy, using his one - punch power to turn the tides of the battle in unexpected ways.
One idea could be a crossover where Saitama from One Punch Man meets Tanya from Youjo Senki in a strange new world. Maybe they have to team up to fight a common enemy that has powers from both of their universes.
Another fanfiction idea could be that Tanya somehow gets the power of Saitama. She then has to deal with the implications of having such overwhelming strength in her military - dominated world. It would be interesting to see how she uses it and how it affects her relationships with her comrades.
Genos from One Punch Man is also quite popular. He could be paired with Tanya in a fanfiction, perhaps as they compare their combat styles or technological enhancements. Also, in Youjo Senki, Viktoriya Ivanovna Serebryakov is sometimes used in fanfiction as she has a strong relationship with Tanya and could add more depth to a crossover story.
Well, you could write a fanfiction about a new hero who joins the Hero Association and has a unique power that somehow challenges Saitama's world view. For example, a hero who can control time in a very limited way but uses it in the most strategic manner. Or perhaps a story where all the villains team up against the heroes and Saitama has to face an army of powerful foes all at once. It would be interesting to see how he handles that without just one - punching everyone immediately.
A crossover with 'Dragon Ball' would be cool. Picture Saitama facing off against Goku. It would be an interesting contrast between Goku's love for a good fight and Saitama's boredom with opponents who can't challenge him.
Perhaps Si could be a new hero who has the power to absorb the powers of defeated monsters. This would make him a very controversial figure among the hero community. Some would see him as a threat, while others would recognize his potential to become one of the strongest heroes. He could also have a complex backstory related to a secret experiment that gave him this power.
One interesting idea could be exploring the backstories of the S - Class heroes in more depth. Maybe there are hidden pasts or traumas that made them who they are in the One Punch Man world. Another idea could be creating new villains that are not just powerful physically but also have complex motives and plans that pose a real threat to the heroes.
A great fanfiction idea could involve the ninjas of Naruto trying to figure out how Saitama got his incredible strength. They might study him in the hopes of creating their own super - soldiers. Another idea is a time - travel fanfic where Saitama goes back in time to the founding of the ninja villages in Naruto. And there could be a story where the One Punch Man characters get chakra and start using ninja techniques in their own battles.
One idea could be that Ben Tennyson gets the power of One Punch Man. He then has to deal with his normal alien - fighting foes but with this new overpowered ability. It would be funny to see how he adjusts to being able to defeat enemies with just one punch while still trying to use his Omnitrix in creative ways.
A cool concept for a RWBY and One Punch Man fanfiction would be a crossover tournament. Characters from both universes are invited to participate in a fighting tournament. Blake could use her speed and agility to surprise some of the One Punch Man characters, while characters like Mumen Rider could show their unwavering spirit in the face of the RWBY combatants. Also, Yang's powerful punches could be compared to Saitama's, and there could be a friendly rivalry between them over who has the stronger punch. This would lead to some interesting interactions and battles within the fanfiction.
One idea could be a yandere Genos who becomes overly possessive of Saitama. He might start interfering in Saitama's daily life, like trying to keep other heroes away from him under the guise of protecting Saitama's secret strength. Another idea is a yandere villain who falls in love with Saitama after being defeated by him and then starts stalking him, determined to make Saitama hers.
How about a crossover with 'Attack on Titan'? Saitama could find himself in the world of the Titans. His ability to defeat any enemy with one punch would be extremely useful against the Titans. He might not understand the complex politics and fear that surrounds the people in that world at first. But as he starts taking down Titans left and right, he would become a sort of legend among the survivors.