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.
Another fanfiction concept could be that the monsters in the 'Danmachi' dungeon start to mutate in a way similar to how the monsters in 'One Punch Man' are strange and powerful. This would force the adventurers in 'Danmachi' to come up with new strategies, and perhaps some of the heroes from 'One Punch Man' could come to help out, like Genos, who could provide some technological insights on dealing with these new threats.
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.
One possible scenario in Batman and One Punch Man fanfiction could be a battle of wits between Batman and Saitama. Batman, with his strategic mind, would be intrigued by Saitama's overwhelming strength. Saitama, on the other hand, might be confused by Batman's complex gadgets and plans.
A fanfiction could show Saitama getting lost in Gotham. Batman finds him and is immediately on guard because of Saitama's strange appearance. But when Saitama just asks for directions in his usual bland way, Batman is a bit taken aback.
In fanfiction, Lord Boros can also be a character that bridges different story arcs. His presence can attract other powerful beings from across the universe. This can lead to more complex and intertwined storylines. Writers might use him to introduce new concepts like different forms of power or alien politics. His backstory can be further developed, like exploring his rise to power in his home planet and how that influenced his actions on Earth.