One idea could be that Saitama from One Punch Man somehow ends up in the world of Overlord. He might be mistaken for a powerful player at first. Since he's so strong, he could easily defeat many of the powerful monsters in the Overlord world, which would cause a stir among the various guilds and NPCs. Another plot could involve the characters from Overlord trying to understand Saitama's power, which is beyond their comprehension as it's not based on magic or typical RPG stats.
Perhaps in the fanfiction, a character from Overlord, like Ainz Ooal Gown, decides to visit the world of One Punch Man out of curiosity. He is shocked by the simplicity of the world's power structure compared to his own. Meanwhile, the heroes in One Punch Man are initially suspicious of this strange, powerful being. As time passes, Ainz starts to interact with the heroes, and they begin to learn from each other. Saitama might be the only one not overly impressed by Ainz's power, which could lead to some comical and interesting interactions.
A plot could be that the heroes from One Punch Man and the powerful beings in Overlord form an alliance to face a common threat. For example, a new entity that is even more powerful than the strongest in both worlds. The heroes and the Overlord characters would have to learn to work together, despite their different power systems and personalities. Saitama could be the key to their victory, but they first need to figure out how to use his power effectively in a coordinated way.
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.
A plot could involve a villain in the DoS (Darkness or Shadow) realm who has the ability to nullify Saitama's strength for short periods. This villain is causing chaos in the city, and Saitama has to figure out how to defeat him without relying solely on his brute force. Maybe he teams up with other heroes who have skills in detecting and countering such abilities.
In a One Punch Man Blizzard fanfiction, a great plot could be that Blizzard is challenged by a rival group who claim to be stronger. Saitama, in his boredom, decides to help Blizzard prove them wrong. He does this in the most nonchalant way possible, like just one - punching all the opponents while eating an ice - cream. This makes Blizzard both frustrated and in awe of him. Another idea is that Blizzard discovers a hidden power within herself after being inspired by Saitama's confidence in his own strength.
In a One Punch Man fanfiction, a great plot could be the origin story of some of the minor heroes. For example, how Mumen Rider got his bike and started his hero journey. Another plot could be a tournament among the heroes to see who is the most skilled (not the strongest since that's Saitama). And a story where Saitama has to deal with a PR disaster for the Hero Association because of his non - traditional hero behavior.
Another plot could involve Sii being a civilian who accidentally gets caught in the middle of a fight between Saitama and a villain. Sii then discovers they have a latent psychic ability that they use to help Saitama in the fight, and this leads to Sii becoming an apprentice of sorts to Saitama, learning about being a hero.
One idea could be Issei getting transported to the One Punch Man universe and trying to prove his worth among the powerful heroes there. He might start by challenging some of the lower - ranked heroes and learning the ropes of this new world.
One plot could be Tornado having to team up with a new, unexpected hero to face a powerful villain that even she has trouble with. This would show a different side of her as she has to work with someone she might not initially get along with.
A plot could involve a villain who has the power to control hair and targets Saitama because his hair is so unique. This leads to a battle where Saitama has to figure out how to fight without using his hair as a weapon, which is quite difficult since his every move usually involves his head (and thus his hair) in some way.
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 idea could be an average guy suddenly gaining a power similar to Saitama's but in a very limited way. He tries to use it to help in small ways around his neighborhood, like stopping petty thieves or helping old ladies cross the street. But he constantly struggles with not being able to fully control it, leading to some comical situations.