One common theme is friendship evolving into something more. They often start as friends or classmates and then gradually develop romantic feelings for each other.
One popular fanfiction with this concept might be 'Twin Flames' (this is a made - up title for example). In it, Izuku being Todoroki's twin sister adds an interesting dynamic to the story. It often explores how their different upbringings, with Izuku in a more normal environment initially compared to Todoroki, shape their personalities and relationships within the UA world.
There's 'Shared Dreams'. In this fanfiction, izuku and fem todoroki are paired up for a special project at school. As they work on it, they start to understand each other better. It's a really heart - warming story with a lot of cute moments between them.
Family is a big theme. Since they are twins, it explores how they are related in the Todoroki family dynamic. There could be issues like acceptance, especially if Izuku was previously unaware of her relation to the Todorokis.
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.