One common theme is growth. Both characters often grow as individuals and in their relationship. For example, they might learn to overcome their insecurities or past rivalries. Another theme is the power of friendship evolving into love. They start as classmates or rivals, and then their bond deepens into something more romantic.
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.
One popular 'deku x female kacchan fanfiction' might be 'A New Bond'. In this story, Deku and female Kacchan start to see each other in a new light after a shared adventure. Their relationship develops as they face challenges together and learn to understand each other's strengths and weaknesses.
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.
To write this fanfiction without the 'lemon' aspect, think about their backstories. Deku's fall into villainy could be due to some great betrayal or a different view on hero society. Todoroki might be on a mission to either save or stop Deku. You can build a story around their ideological differences and how they interact with each other. For example, they could have a series of battles or encounters where they gradually understand each other more.