I'm quite sure there exists a 'Naruto is One Punch Man' fanfic. The world of fanfiction is vast and full of all kinds of creative crossovers. Writers might be intrigued by the idea of bringing Naruto's ninja skills and the unique world of One Punch Man together. For example, how would Naruto react to the overpowered villains in One Punch Man's world? And how would Saitama view Naruto's determination to protect his village? These are the kind of things that could be explored in such a fanfic.
Another idea is that Naruto, curious about other powerful beings, uses a jutsu to travel to the One Punch Man universe. There, he would be amazed by Saitama's strength. Naruto could try to learn from Saitama how to be so effortlessly strong, but Saitama would probably just shrug it off and say there's no special trick. They could team up to fight some of the monsters in Saitama's world, with Naruto using his ninjutsu and Saitama just punching things away.
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, Naruto's shadow clone jutsu could be really useful in a fight alongside Saitama. The clones could distract enemies while Saitama gets in a powerful punch. And Saitama's super strength could help Naruto in situations where brute force is needed more than ninjutsu. For example, if they had to break through a really thick and magically reinforced wall.
One interesting concept could be the combination of One Punch Man's overpowered characters with other universes' power systems. For example, if Saitama were to enter the Dragon Ball universe, how would his 'one - punch' ability interact with ki - based powers? It would create a really unique dynamic where his seemingly unbeatable strength might be challenged in new ways.
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.
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.
Saitama is by far the most popular. His unique power of being able to defeat any opponent with one punch makes him a fascinating character to insert into other universes. Whether it's facing off against powerful foes in Naruto's ninja world or the superpowered individuals in My Hero Academia, his presence always shakes things up.
There are some dedicated anime and manga fanfiction forums where you might find good 'One Punch Man Crossover Fanfiction'. For example, Reddit has several sub -reddits related to anime fanfiction. You can post a request for 'One Punch Man' crossovers there or just search through the existing posts. Some Tumblr blogs also share and recommend fanfiction, so it's worth checking out Tumblr too.