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.
I'm not sure there's a direct relationship. 'Onepunch Man' manga is a popular series on its own, and 'Baka' might not have a specific connection to it.
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.
One Punch Man is a really popular anime. Fubuki, as a character in it, has a lot of potential for fanfics. In a fanfic about them, it could explore their relationship in new ways. Maybe it could show Fubuki trying to recruit Saitama into her group in a more detailed and different way than in the original story. It could also develop her character further by showing her reactions to Saitama's overpowering strength in unique scenarios.
The fanfiction about Fubuki and Tatsumaki in One Punch Man is alluring because it gives a chance to expand on their characters. Fubuki, with her ambition to be a great hero, can be explored further. Maybe in a fanfic, she overcomes her insecurities related to her sister Tatsumaki's much stronger powers.
One possible plot could be that Fubuki, being frustrated with her lack of progress in terms of power compared to Saitama, decides to train with him. This would lead to a series of comical and at the same time, character - building moments. Saitama would probably give her some really simple but effective training advice that she initially scoffs at but later realizes is genius.
Many One Punch Man fanfics with Fubuki and Tatsumaki focus on their interactions with other characters too. They could be paired with Saitama in some comical or action - packed situations. Like, Fubuki trying to recruit Saitama into her group while Tatsumaki is constantly skeptical of his abilities.
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.