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.
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.