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.
A 'villain deku x villain todoroki fanfiction' might focus on their backstories that led them to become villains. It could be that they were both wronged by the hero system in some way. Deku might have been rejected too many times or faced unfair treatment, and Todoroki could have some family - related issues that pushed him to the dark side. The story could then show how they form an alliance and start their villainous journey together.
The 'Villain Todoroki x Deku' fanfiction might also focus on power dynamics. Since both characters have strong quirks, how they use or misuse their powers in relation to each other can be fascinating. For example, Todoroki's ice and fire powers could be used in more menacing ways as a villain, and Deku would have to find new strategies to counter him while still trying to save him from his villainous ways.
Perhaps they get involved in a power struggle within the villain world. They want to prove themselves as the most feared villains. This could lead to battles with other villains and alliances with some unexpected characters. It could also involve them trying to steal some powerful artifacts or secrets from the hero side to gain an edge in their villainous activities.
The appeal of 'Villain Todoroki x Deku' fanfiction lies in the contrast between the two characters. Deku is the epitome of a hero, while a villain Todoroki goes against his original heroic - inclined nature. This contrast creates a lot of tension and potential for interesting storylines.
I don't encourage seeking out 'lemon' fanfiction which often contains sexual content. But if you want to explore the relationship between Villain Deku and Todoroki in a more appropriate way, you can join fan forums and ask other fans if they know of any well - written stories that focus on their dynamic, be it a more adversarial or a complex relationship that doesn't involve adult themes.
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.
One possible plot reason could be envy. Maybe Todoroki, as a villain in this fanfiction, is envious of Deku's popularity and the support he gets from fans. So, he kidnaps Deku to take away that source of admiration from him. Another reason might be related to power. If Todoroki has turned villain, he could see Deku as a threat to his plans, and by kidnapping him, he aims to neutralize that threat.
Midoriya's friends like Uraraka and Iida would be extremely worried. They would immediately start looking for him. Uraraka might use her quirk to search in the air, while Iida would lead the ground search with his speed. Bakugo, on the other hand, would be furious. He would probably go on a solo mission to find Deku and take on Todoroki without hesitation.
Common themes include the exploration of their individual insecurities. For female deku, it might be about not being strong enough at first, and for todoroki, his family issues. As they come together, they help each other deal with these. Also, the theme of finding a true connection in a world full of chaos is often there. Their relationship becomes a sort of haven in the midst of all the hero - related chaos. There's also the theme of acceptance, accepting each other's flaws and strengths.