One way could be that Doctor Doom's power lies more in his intelligence and use of technology and magic. So, he could find ways to level the playing field against Saitama. For example, he could create power - nullifying fields or devices. Saitama, on the other hand, might not fully understand the nature of Doctor Doom's power, which could also add an interesting dynamic.
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.
One Punch Man is about a guy named Saitama who can defeat any enemy with just one punch. But despite his amazing power, he struggles with the boredom of having no real challenge.