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.
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.
If the grandfather is very warm and accepting, Shikaneji would feel relieved and happy. They might open up more about their relationship and their dreams. Nejire could start chatting excitedly about their future together, and Shinsou would relax and join in the conversation more. However, if the grandfather is not so friendly, they would be disappointed and perhaps a bit defensive.
Well, Bakugo might scoff at first. He'd probably think Batman is too brooding and not as straightforward as he likes his heroes. But then as he reads more of the fanfiction, he might grudgingly admit that Batman's detective skills are cool. Deku, on the other hand, would be super interested. He'd be analyzing every move Batman makes in the fanfiction, taking notes in his head about Batman's strategies and how they could be applied in his own hero work. Uraraka would be more focused on the relationships in the fanfiction. She'd be all 'aww' at the cute moments and feel for the characters when they're in trouble.
An AU fanfic is a type of fan fiction where the story takes place in an alternate universe with different settings or circumstances than the original work.
An au fanfic is a type of fan-written story that takes the characters or setting of an existing work and places them in an alternate universe or situation that differs from the original.