Rimuru Tempest is extremely powerful in the manga. He gains strength by absorbing other creatures' skills and abilities and becomes a force to be reckoned with. His magical powers and strategic thinking make him a formidable character.
Rimuru Tempest has a wide range of powers. One of his main abilities is the power of Gluttony. This allows him to devour almost anything and gain knowledge or abilities from it. For example, he can devour monsters and acquire their skills, which makes him extremely versatile in combat.
The power of Gluttony stands out. It lets Rimuru consume all kinds of matter and energy. He can eat magical items and extract their essence, which he can then use for his own growth or to create new things. This power is fundamental to his development throughout the light novel.
The villains would have diverse reactions too. Garou might see the fanfic as a source of inspiration for his own 'evil' plans. He might think that some of the ideas in the fanfic could make him an even more formidable foe. However, if the fanfic makes him look like a complete fool, he'd be enraged. Deep Sea King would be confused by the fanfic, not really understanding the concepts that are different from his simple view of the world. But he might also be intrigued by the new versions of himself presented in the fanfic.
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.