A cool plot could be that the Dragon Balls are scattered across the One Punch Man world. Goku travels there to find them, and he meets Saitama. Saitama is initially unimpressed by Goku's search for the Dragon Balls, thinking it's a silly quest. But as they interact more, they learn from each other's fighting styles. Meanwhile, villains from both universes start to target Goku and Saitama, forcing them to work together to protect both worlds.
Perhaps a plot where Beerus, the God of Destruction in DBZ, senses an unusual power in the One Punch Man world. He sends Goku and Vegeta to investigate. There, they meet Saitama. Saitama doesn't understand their talk about ki and Saiyans at first. As they face common enemies, Saitama shows his incredible strength, and Goku and Vegeta try to figure out how he can be so powerful without ki. Eventually, they all join forces to stop a threat that could destroy both universes.
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 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.
It's really hard to say. Saitama is known for being able to defeat any opponent with one punch in his own universe. But in the DBZ universe, characters like Zeno can erase entire universes. However, Saitama's power seems to be limitless in a different way. He doesn't seem to have any physical limits like the Saiyans do. So it could be Saitama if his power translates well into the DBZ universe, or it could be Zeno who has such a high - level power over existence.
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.
Well, in a 'Dbz Captain America crossover fanfiction', perhaps there's a threat that endangers both universes. Captain America's sense of justice and unwavering moral compass could inspire the Z - Fighters. They could learn from his strategic thinking. And Captain America could be amazed by the power of ki and the Saiyans' transformations. Maybe they start training together, with Captain America trying to keep up with the intense training of the Z - Fighters, and in the end, they save both universes.
For a 'dbz gohan crossover fanfiction', a plot where Gohan crosses into the DC Comics universe would be exciting. He could team up with Superman and learn about different types of powers. Maybe he'd help Superman understand the concept of ki. Another idea is a crossover with the Final Fantasy series. Gohan could find himself in a world full of summons and magic, and use his martial arts to fight alongside the Final Fantasy characters.
One plot idea could be that the Dragon Balls are accidentally brought into the Evangelion universe. This causes a stir as different factions in the Evangelion world, like NERV and SEELE, try to get their hands on them for their own purposes. Meanwhile, the Z - Fighters follow the Dragon Balls and end up in this strange new world. Another idea is that an Eva unit gains the ability to use ki, like a Saiyan. This Eva then goes rogue and starts causing chaos, and it's up to both the Evangelion pilots and the Z - Fighters to stop it.
One idea could be that Broly ends up in an alternate universe where he has to team up with Goku and the Z - fighters to fight a new, even more powerful enemy. This would force Broly to control his anger and work with his former foes.