Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.
Some fanfictions might include Uraraka's family members. Her parents could be involved in their married life, for example, giving Ochako traditional family advice. Also, Izuku's mother could be a prominent character, sharing her own experiences of being a single mother and now seeing her son happily married.
One great izuku and ochako fanfiction is 'A New Beginning'. It shows their relationship development in a really sweet way. They face challenges together and support each other, and the author does a wonderful job of capturing their personalities.