I don't know for sure as it depends on the specific fanfic. It could be a main character from the original My Hero Academia series who is put in a new and difficult situation that makes them 'broken but not defeated'.
I'm not sure as I haven't read it, but it could be the black fox character if there is one. Maybe some original characters created by the fanfic writer who interact with the MHA canon characters.
A likely main character from MHA could be Midoriya. His determination and growth throughout the series make him a great candidate to interact with the daughter of Poseidon. And of course, the daughter of Poseidon herself would be a main character. She would bring her own set of powers and a different perspective from the world of the gods.
All Might could play a significant role too. His status as a symbol of peace in MHA and his experience would be valuable. In the Super Sentai side, the mentor - like figure or the Green Ranger might be the counterparts. All Might could share his wisdom with the Super Sentai team while learning some new tricks from them as well.
Spiderman has his super strength, agility, and the famous spider - sense. In comparison, MHA characters have a wide variety of quirks. For example, Todoroki has his ice and fire powers. Spiderman's agility might be on par with some of the more acrobatic MHA characters like Sero. But his spider - sense gives him an edge in combat that most MHA characters don't have. However, quirks like All Might's super strength can rival Spiderman's own strength.
Deku is quite popular. His journey to become a great hero can be easily translated into a gaming context, like him leveling up his skills. Bakugo is also a favorite. His brash personality and powerful quirk make for exciting scenarios in fanfictions.
One of the main themes is heroism. It shows different characters' concepts of being a hero, like Izuku Midoriya's journey to become a great hero. Another theme is friendship, as the characters support and grow together. And there's also the theme of self - discovery, where characters learn about their own powers and identities.
Izuku Midoriya is really popular. His journey from being a quirkless boy to a powerful hero - in - training is very inspiring. Katsuki Bakugo is also popular. His explosive personality and strong power make him stand out. And Ochaco Uraraka with her cute appearance and kind heart is liked by many.
Uraraka would probably find it really interesting. She has a positive and curious nature. She'd be excited to see how the changes in backstories affected the relationships between the characters. And she might even share her thoughts about it with her friends in the MHA universe.
Some MHA characters might be really intrigued by Disney fanfiction. For example, Izuku Midoriya would probably be super excited. He loves stories and analyzing different characters' motives, just like he does with heroes. He'd be all about exploring the new worlds and character relationships in the Disney fanfics.