Well, in such a fanfiction, it often explores a different side of Deku being a villain and his relationship with Todoroki. It could be about how their dynamic changes from the original story. Maybe Deku as a villain has different motives and goals that somehow involve Todoroki, like trying to use Todoroki's power for his own evil plans or trying to turn Todoroki to his side.
Power imbalance is often a theme. Deku might gain new, more dangerous powers as a villain, and Todoroki has to figure out how to deal with that. There could also be a theme of trust. Since Deku is a villain, Todoroki has to decide whether to trust him or not. And if he does trust him, it could lead to all sorts of complications in the story.
Once upon a time, there was a Todoroki sister OC named Hana. She had the same white and red hair as her brother Shoto. Hana was often in the shadow of her powerful siblings. But she had a unique power of her own - the ability to manipulate ice and fire separately, not just together like Shoto. One day, she decided to prove herself. She entered a hero competition at school. At first, everyone underestimated her. But as the challenges went on, she showed her true strength, impressing everyone with her precise control over her powers.
One common theme is rivalry. Bakugo's aggressive and competitive nature against Todoroki's more reserved yet powerful self makes for an interesting dynamic. Another theme is friendship development, as they often start off at odds but then find common ground.