Deku would likely become more confident in his abilities. In the One Punch Man universe, he'd see Saitama's nonchalant attitude towards fighting and might start to adopt a similar mindset. He'd also have to adapt to the different types of villains. Since One Punch Man villains are often very strange and powerful in unique ways, Deku would need to be more creative with his strategies.
One popular aspect in One Punch Man subterranean fanfiction is the re - imagining of the subterranean races. Writers often give them more complex backstories and motives. For example, some fanfics might depict a subterranean character who has a change of heart and decides to side with the surface - dwellers. This adds an interesting layer of depth to the story as it challenges the traditional view of the subterranean as simply enemies in the original work.
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.
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.