Sure, Deku takes on the role of a vigilante in the manga. It's a bold move that showcases his growth and the challenges he faces outside the conventional system of heroes.
One common theme is the struggle for justice outside the traditional hero system. Vigilantes in these fanfics often feel that the official heroes are not doing enough or are too restricted by the law, so they take matters into their own hands.
In fanfiction, a quirkless Deku often has to rely more on his wits. Canon Deku got One For All, but in fanfic, he has to find other ways to be strong. For example, he might study different fighting styles more intensively.
Toga could be an ally. Since she has her own complex relationship with the idea of heroes and villains, she might find Deku's transformation interesting. She could provide some of her unique skills, like shape - shifting, to help Deku in his villainous plans. Additionally, Twice might also join them because of his loyalty to Toga and his own sense of being an outcast in the hero world.
First, for your My Hero Academia Vigilante OC in fanfiction, consider their motivation. It's important. Are they fighting for revenge, or to protect a certain group of people? For example, if they had a family member who was wronged by a villain and the heroes didn't do enough, that could be a strong motivation. Next, when it comes to their skills, don't just rely on their quirk. They should have some combat training or street - smart skills. Like being able to quickly assess a situation and come up with a plan. And finally, their relationships with other characters can add depth. Maybe they have a begrudging respect for some heroes while being at odds with others.
It could be a powerful villain who sees Naruto as a threat to his plans in the My Hero Academia world. Maybe a villain who wants to use Naruto's unique abilities for his own evil purposes and when Naruto refuses, becomes his enemy.
There's also 'Growth Together'. This fanfic shows how Deku and Todoroki support each other during tough times at UA. They face challenges not only from villains but also from within the school. Their relationship develops as they help each other overcome these obstacles.