The concept of a hero who can defeat any enemy with one punch is very unique. It subverts the traditional idea of long, drawn - out battles in superhero stories.
Tatsumaki, also known as Tornado of Terror, is a powerful esper in the novel. She has psychic powers and a rather arrogant personality. She often gets involved in various battles against powerful monsters, and her relationship with other heroes, especially Saitama, is quite interesting as she initially looks down on him.
One great MLP and Onepunch Man fanfiction is 'The Pony Puncher'. It combines the cute and colorful world of MLP with the overpowered and comical nature of Onepunch Man. The story has Saitama somehow ending up in Equestria and his reactions to the magical ponies are hilarious.
Female Genos could be developed with a more empathetic side. Since she is a cyborg, she could struggle with understanding human emotions more deeply. Maybe she starts to learn about love and friendship in a different way compared to male Genos. For example, she might be more sensitive to the feelings of other female characters around her.
Another important aspect is world - building. Since you're combining two different worlds, you need to figure out how they interact. Do the ponies understand Saitama's world? How does the power system in Onepunch Man work in the context of Equestria? You can also look at other fanfictions for inspiration, but make sure to put your own spin on it. Write regularly and don't be afraid to revise your work as you go along.
Genos is still kicking in the One Punch Man manga. The author has managed to keep him in the storyline without killing him off. There's always the possibility in the future, but for now, he's alive and fighting.
One way is to put Genos in a situation where he's outnumbered. As the enemies attack him from all sides, his defenses are slowly broken down. The writer can describe the attacks hitting him one by one, each causing more damage. They can also make the reader feel Genos' desperation as he realizes he might not be able to survive this time. And then, there's the aftermath. How does he deal with the pain and the damage? Does he become more determined or does he start to doubt himself? These are all aspects that can make a hurt scene for Genos impactful in a fanfiction.
First, consider the role the OC will play. If it's a partner for Genos, think about their combat abilities. For example, if Genos is mainly a physical fighter, the OC could be a support with long - range attacks or healing powers.