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.
Maybe there's a threat in the Splatoon world that the inklings and octolings can't handle. They hear rumors of a super - powerful being in another dimension (Saitama's world). So they manage to open a portal and go to Saitama's world to ask for his help. Saitama is reluctant at first but then decides to go with them. In the Splatoon world, he has to adjust to fighting with ink and using the unique weapons there. His one - punch power still works wonders, and he helps the inklings and octolings defend their world, and in the process, he discovers new ways to have fun.
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.
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.
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.
In a crossover, Saitama might be the odd one out in the colorful and ink - filled Splatoon world. But his simplicity and straightforwardness would attract the inklings. They would try to get him to embrace their culture, like trying different ink colors. Saitama would just go with the flow. Genos, on the other hand, would be more focused on how the Splatoon world could help him become stronger. He would observe the inklings' and octolings' teamwork during battles. The inklings and octolings would be fascinated by Genos' loyalty to Saitama and might try to understand the concept of a master - apprentice relationship in the context of their own social structures.
One interesting plot could be that Bowser teams up with the Octarians in the Splatoon world. He wants to use their ink technology to take over both the Mushroom Kingdom and the Splatoon world. Mario has to then form an alliance with the inklings. He travels through different ink - filled levels, learning new skills like swimming in ink and using ink - based weapons. Along the way, he discovers secrets about the connection between the two worlds. They finally face Bowser and the Octarian leader in a huge battle where Mario combines his classic moves with the ink - powered attacks of the inklings to save both worlds.
One possible plot could be that a new and extremely powerful villain emerges who is not impressed by Saitama's one - punch ability at first. This villain manipulates the rules of the tournament to his advantage, creating unfair match - ups for the heroes. The heroes then have to find a way to overcome these obstacles while also dealing with their own rivalries and personal growth. For example, Genos could be paired against a much stronger opponent early on, and in the process of trying to defeat this foe, he discovers a new power - up within himself.
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.