Yes, there likely is. The Naruto and One Punch Man fandoms are both quite large, and it's common for fans to create crossover fanfictions. These crossovers can be really interesting as they combine the unique elements of both universes, like Naruto's ninja skills with Saitama's one - punch power.
Similarities? Well, both are super - powerful. They both are often the ones to save the day in their respective universes. Differences? One Punch Man's power is very straightforward - that one punch. Superman has a whole range of powers. In fanfiction, One Punch Man might be more of a comical character in his overpoweredness, while Superman is more of a traditional hero figure.
The concept could be expanded by giving the punch different properties. For instance, it could have the ability to nullify an enemy's special powers instead of just knocking them out physically.
In some fanfictions, the strongest punch could have a different impact on Saitama himself. Instead of being a simple display of his overpowered strength, it could start to drain his energy in a new way. This would add an interesting twist to his character, as he would have to be more cautious about when and how he uses his ultimate move.
In a battle between One Punch Man and Superman, it's really tough to say. One Punch Man can defeat any opponent with just one punch. But Superman has incredible strength, speed, and a whole bunch of other powers like heat vision and flight. If One Punch Man managed to land his one punch, he could win. However, Superman's speed might make it difficult for One Punch Man to even get a hit in. So, it could go either way depending on the circumstances.
The fanfiction about Fubuki and Tatsumaki in One Punch Man is alluring because it gives a chance to expand on their characters. Fubuki, with her ambition to be a great hero, can be explored further. Maybe in a fanfic, she overcomes her insecurities related to her sister Tatsumaki's much stronger powers.
I've read a really interesting One Punch Man Rover fanfiction where Rover teams up with Saitama in a really unexpected way. They end up facing a new type of monster that can manipulate time, and Rover's unique abilities play a crucial role in the battle. You can often find such gems by simply searching for 'One Punch Man Rover' on different fanfiction platforms and sorting through the results.
One great Haikyu and One Punch Man fanfiction could be where the characters from Haikyu find themselves in the world of One Punch Man. They might be shocked by Saitama's incredible strength. Another could be a crossover where the volleyball skills of the Haikyu characters are used in a unique way in the One Punch Man universe, like to defeat some of the monsters in a really creative way.
One possible fanfiction could be about the Metal Bat facing an extremely powerful and unknown monster in the One Punch Man universe. He could struggle in the fight at first, using his bat with all his might. But as the battle progresses, he discovers a new level of strength within himself, inspired by the other heroes he has seen in action.