Genos from One Punch Man himself would be great. He could be the one who tries to explain Saitama's power to the characters from the other crossover universe. For example, in a crossover with Attack on Titan, Genos could be the one to tell the Survey Corps about Saitama's invincibility as they face the Titans.
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.
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.
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.
A fanfiction might center around a competition between Popeye and Saitama. They could have a friendly contest to see who can defeat the most villains in a day. Popeye would use his wits and spinach - enhanced strength, while Saitama would rely on his incredible punching power. This could lead to some hilarious and exciting encounters as they both try to outdo each other.
There's 'Saitama in Remnant' which is also a popular one. In this fanfic, Saitama finds himself in the RWBY world. His overpowered abilities create a lot of chaos and also some hilarious moments as he tries to understand the different rules and threats in Remnant. It also has some great fight scenes where his simple punch causes massive destruction in the RWBY setting.
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 Punch Man is a really popular anime. Fubuki, as a character in it, has a lot of potential for fanfics. In a fanfic about them, it could explore their relationship in new ways. Maybe it could show Fubuki trying to recruit Saitama into her group in a more detailed and different way than in the original story. It could also develop her character further by showing her reactions to Saitama's overpowering strength in unique scenarios.