Well, a possible plot could be that Doctor Doom, believing he is the most powerful in his own universe, challenges Saitama to a fight. But of course, Saitama's nonchalant attitude towards the fight annoys Doctor Doom even more. As the story progresses, Doctor Doom realizes that Saitama's power is not something he can simply understand or defeat with his usual tactics. It could lead to a character - growth arc for Doctor Doom where he has to re - evaluate his concept of power.
One idea could be that Doctor Doom, intrigued by Saitama's power, tries to find a way to steal it. He might use his advanced technology and magic to create a device that can absorb Saitama's strength. Another plot could involve them teaming up against a common enemy that threatens both their worlds. Maybe an interdimensional being that is causing chaos in both the Marvel universe and the One Punch Man universe.
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 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.
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 really interesting fanfiction plot could be about a group of scientists who were working on a project to reverse - engineer the demon technology. They think they can use it to create a weapon against the demons. However, things go wrong and they accidentally open a new portal to a different demon dimension, which brings in even more powerful and unknown demons. The story could then focus on how they deal with this new threat and try to close the portal.
One way could be that Doctor Doom's power lies more in his intelligence and use of technology and magic. So, he could find ways to level the playing field against Saitama. For example, he could create power - nullifying fields or devices. Saitama, on the other hand, might not fully understand the nature of Doctor Doom's power, which could also add an interesting dynamic.