One common theme is healing. Rei has been through a lot, so in many fanfics, the reader is there to help her heal emotionally. For example, the reader might be the one to encourage Rei to open up about her past traumas.
There's 'Rei's New Friend' which is also quite popular. In this fanfic, the reader enters Rei's world in an unexpected way. It shows how the reader's presence gradually changes Rei's life for the better. The writing style is very engaging, making you feel like you're really part of the story.
The main characters would obviously be Izuku Midoriya and Rei Todoroki. Izuku is the protagonist of the main series, a quirkless boy who dreams of becoming a hero. Rei is Shoto Todoroki's mother, who has her own complex backstory and powers. In a fanfiction, they would be the central focus, with their interactions and character development driving the story forward.
One possible plot could be that Izuku discovers Rei's hidden strength. Maybe Rei has some untapped quirk potential that Izuku, with his analytical mind, helps her realize. They could team up to face a new threat in the My Hero Academia world that requires both of their unique skills and experiences.
His friends would probably take care of him. Midoriya would be very eager to help, using his kind-hearted nature. He would make sure Todoroki is fed and comfortable.
One idea could be that all his friends at UA plan a surprise birthday party for him. They decorate the common room with his favorite colors, blue and red. Another plot could be Todoroki spending his birthday with his family, trying to reconcile with his father on this special day. Maybe he receives a heartfelt gift from his mother that has a deep emotional significance.