In a One Punch Man DC crossover fanfiction, a great plot could involve Dr. Eggman from the Sonic the Hedgehog universe (just kidding, wrong universe, let's start again). A good plot could be that Darkseid discovers Saitama's world and decides to invade it. Saitama, of course, is not impressed. He starts punching Darkseid's minions left and right. This draws the attention of the DC heroes, who come to help. But they soon realize that Saitama is on a whole different level of power. The story could then focus on how they all learn to work together, with Saitama's simplicity and overpowered nature causing some comical and also some very interesting strategic discussions among the heroes.
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.
Saitama and Batman would be really interesting. Batman is a normal human with no superpowers but incredible skills and intelligence. Saitama is the opposite, having unlimited power but a rather simple - minded approach. Their interaction would be full of contrasts, like Batman trying to analyze Saitama while Saitama is just looking for a good fight or a cheap sale at the supermarket.
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.
One interesting 'One Punch Man DC Fanfiction' could be a crossover where Saitama meets Superman. They might have a friendly sparring match that ends up with Saitama's nonchalant punches and Superman's super strength creating some hilarious and unexpected situations. Another could be where Genos teams up with Cyborg to fight a common enemy. Their different robotic enhancements and combat styles would make for an exciting story.
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.
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.
Perhaps in the fanfiction, Spider - Man's sense of humor clashes with the more serious tone of the DC heroes at first. But then they realize they can learn from each other. For example, Superman could teach Spider - Man how to better control his strength, while Spider - Man shows the DC heroes how to have a more light - hearted approach in the face of danger. And they all come together to fight a common enemy like Darkseid who has somehow managed to get his hands on some of Spider - Man's tech.
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.
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.