In a One Punch Man and Avengers crossover fanfiction, the plot could center around a cosmic event that merges their two worlds. Saitama finds himself in New York and starts to interact with the Avengers. Iron Man might be intrigued by Saitama's power and try to study him. Meanwhile, villains from both universes could team up. Saitama's nonchalant attitude towards the powerful foes would contrast greatly with the Avengers' more serious approach. Eventually, Saitama would defeat the main villain with his one punch, but not before some hilarious and action - packed scenes between him and the Avengers.
One possible plot could be that Saitama somehow ends up in the Marvel universe. He might be mistaken for a new superpowered being by the Avengers. Maybe he just casually punches a big bad like Thanos during one of his attacks, completely oblivious to the whole situation, leaving the Avengers both shocked and amused.
The plot could involve Garou somehow ending up in Saitama's world more permanently. Maybe he decides to team up with Saitama against a common enemy. This new enemy could be a powerful being from another dimension that neither of them could face alone. Their contrasting fighting styles would make for an interesting dynamic, with Saitama's one - punch power and Garou's martial arts skills.
One idea could be that Saitama somehow gets transported to the DC universe. He might initially be confused by all the superpowered beings there. Maybe he enters a hero - villain battle in Gotham and just one - punches all the villains, which both amazes and confounds the local heroes like Batman. They start to investigate this strange, bald - headed, extremely powerful man.
One possible plot could be that Saitama somehow ends up in the Dragon Ball Z universe. He meets Goku and they decide to have a friendly sparring match. But when Saitama throws his punches, Goku is shocked at the power, yet also realizes Saitama has no ki. Then they team up to face a new, super - powerful villain that threatens both their worlds.
A plot could involve an experiment gone wrong in the One Punch Man world that creates a rift, pulling in inklings and octolings. Genos and Saitama have to deal with these new, ink - based beings. The inklings mistake Saitama for a super - strong boss - like character and try to challenge him. Saitama, of course, defeats them with one punch, but then realizes they're not really bad. So he teams up with them to stop the real threat, which is a mutated sea creature that has been affected by the same experiment and is now attacking both worlds.
One possible plot could be that Saitama somehow ends up in the Naruto universe. He might be mistaken for a very powerful shinobi at first. Naruto, being Naruto, would probably be curious about Saitama's strength and try to befriend him. They could team up to face a new, extremely powerful enemy that neither universe has seen before, like a super - strong demon that can absorb chakra and is also incredibly durable like the monsters in One Punch Man.
The interactions would be quite diverse. Thor might see Saitama as a strange but powerful being. He could offer to have a friendly sparring match, which Saitama might decline because it's too much trouble. Black Widow would likely be trying to figure out Saitama's background and what makes him tick. And Spider - Man would be super excited to meet someone with such incredible power, constantly asking Saitama questions about how he got so strong.
One possible plot could be that Superman finds himself in the Marvel universe due to some sort of interdimensional rift. The Avengers initially see him as a threat. But then they realize he's there to help against a common enemy like Thanos. Superman's powers would be a huge asset, and his sense of justice would align with the Avengers' mission.
The plot could revolve around a villain who targets Saitama's hair specifically. This villain might believe that Saitama's power is directly related to his hair and tries to steal it or cut it off. Saitama then has to protect his hair while also dealing with the normal chaos of fighting monsters in the city.
A plot could involve Saitama ending up in the Naruto world. He quickly realizes that the power levels here are different. He may get involved in the ninja wars, using his one - punch ability to end battles quickly. Naruto, intrigued by Saitama's power, tries to figure out how he can achieve such strength. Meanwhile, the villains in the Naruto world start to fear this new, extremely powerful intruder.
Well, it could be about the Straw Hats accidentally ending up in the Marvel universe. They meet the Avengers and at first there's some misunderstanding because of their different ways. But then they realize they can work together. Maybe Luffy is fascinated by Thor's hammer and tries to lift it, while Zoro wants to have a sword fight with Captain America's shield.