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.
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 there could be a villain in the crossover fanfiction that is a threat to both worlds. Saitama and Ainz have to team up to defeat it. Saitama is used to soloing enemies, and Ainz is used to having his minions do the work. So, they have to overcome their differences in fighting styles and learn to work together. Ainz might be in awe of Saitama's sheer strength, and Saitama might be a bit weirded out by the undead nature of Ainz and his minions.
Another plot could involve a crossover with a different anime universe through a strange portal. For example, if he entered the world of My Hero Academia. Garou could initially try to assert his dominance but quickly realizes that the quirks in that world are different from his martial arts skills. He might then start to learn about these new powers and how to adapt, all while causing chaos and also forming unexpected alliances.
One idea could be a crossover with Dragon Ball. Saitama could participate in the World Martial Arts Tournament and effortlessly defeat all the opponents, creating a hilarious situation. His nonchalant attitude would contrast sharply with the intense and competitive nature of the Dragon Ball characters.
Another idea is that Naruto, curious about other powerful beings, uses a jutsu to travel to the One Punch Man universe. There, he would be amazed by Saitama's strength. Naruto could try to learn from Saitama how to be so effortlessly strong, but Saitama would probably just shrug it off and say there's no special trick. They could team up to fight some of the monsters in Saitama's world, with Naruto using his ninjutsu and Saitama just punching things away.
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.
Well, we could have a situation where the Monster Association from One Punch Man discovers Equestria. They think it's a new place full of potential 'prey'. The ponies, not used to such brutal threats, would be in a panic. Then Saitama follows the monsters to Equestria. Along the way, he befriends some of the ponies, like Pinkie Pie who tries to cheer him up all the time. The ponies and Saitama would then work together to drive the monsters out of their land. It would be a really interesting combination of the two very different universes.
One idea could be that Naruto and Saitama somehow end up in a shared dimension where they have to team up to defeat a common enemy. For example, a powerful demon from Naruto's world that has the ability to regenerate like some of the monsters in One Punch Man's world. Another plot could involve Naruto learning Saitama's training regime and trying to incorporate it into his own ninja training, while Saitama is intrigued by Naruto's chakra system and tries to figure out if he can use it to make his punches even more powerful.
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.