One idea could be a crossover with Dragon Ball. Saitama could participate in the World Martial Arts Tournament and effortlessly defeat all the opponents, creating a hilarious situation. His nonchalant attitude would contrast sharply with the intense and competitive nature of the Dragon Ball characters.
One idea could be that Naruto and Saitama somehow end up in a shared dimension where they have to team up to defeat a common enemy. For example, a powerful demon from Naruto's world that has the ability to regenerate like some of the monsters in One Punch Man's world. Another plot could involve Naruto learning Saitama's training regime and trying to incorporate it into his own ninja training, while Saitama is intrigued by Naruto's chakra system and tries to figure out if he can use it to make his punches even more powerful.
Well, perhaps a plot where Genos follows Saitama into the My Hero Academia world. Genos could enroll in UA as well and show off his advanced technology - based powers. He might get into a competition with Bakugo over who is more powerful. Meanwhile, Saitama is just casually wandering around, accidentally solving problems and leaving the students and heroes in awe of his effortless strength.
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, often a plot point is the culture shock. For example, Saitama being in RWBY's world and not understanding things like Dust or the Grimm. It's funny to see how he just approaches everything with his 'one - punch' mindset. Another is the power - level differences. RWBY characters have their own set of powers which are quite different from Saitama's extreme strength, and seeing how they interact in battles or training is really engaging.
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.
One idea could be that Spider - Man gets accidentally transported to the DC universe. He teams up with Batman to take on a new villain that is a combination of their rogues' galleries. For example, a villain who has the powers of Electro and can also control minds like Brainiac.
In an 'ultimate spider man crossover fanfic', a really interesting crossover could be with Venom. But instead of being enemies, they team up. Maybe there's a new symbiote - like threat that is even more powerful and they have to combine their powers to defeat it. Another idea is a crossover with the Guardians of the Galaxy. Spider - Man gets accidentally transported into space and has to team up with Star - Lord and the gang to get back home while also fighting off some space - pirates.
Well, a cool plot might involve Iron Man being accidentally transported to the Transformers' home planet. There, he has to use his wits and his suit's capabilities to survive. He could end up forming an alliance with some of the Transformers, like Optimus Prime, and they come up with a plan to get him back to Earth while also dealing with a threat to both their worlds. This would involve a lot of cool tech - sharing and strategic planning between the two very different kinds of heroes.