There could be a story where the OC's dark quirk is a secret at first. They are living a normal life until certain events start to trigger the quirk. The fanfiction might explore how they keep this secret from their friends and family. As the story progresses, they might be forced to reveal it when facing a powerful enemy that only their dark quirk can defeat. This could lead to a lot of drama, with their loved ones having to come to terms with this new side of the OC.
Often, there are original characters created specifically for the fanfiction. These OCs are the main focus with their dark quirks. There might also be some canon characters from the original work that the fanfiction is based on. They could be friends, foes or mentors to the OC. For example, if it's a fanfiction based on a superhero story, there could be a well - known hero who tries to help the OC control their dark quirk.
One possible plotline could be that the oc with the evil quirk tries to hide it from others at first. They might be afraid of being ostracized. For example, they could pretend to have a normal quirk in public but struggle internally with the dark impulses their evil quirk brings. Another plotline could involve the oc being forced to use their evil quirk for self - defense in a dangerous situation and then having to deal with the consequences of others seeing what they can really do. Maybe they get hunted down by heroes who misinterpret their actions as villainous.
One interesting concept could be how Izuku's dark matter quirk might give him unique abilities like the power to manipulate gravitational forces. It could also lead to a new form of combat style that's different from the traditional quirks in the My Hero Academia universe.
One idea could be an OC with a sound quirk who can manipulate sound waves to create illusions. For example, in a battle, they make the enemy think there are multiple allies when it's just sound - based images. Another could be an OC that uses their sound quirk to communicate with animals, as different frequencies might be understood by them. A third plot idea is an OC with a sound quirk in a music - based society where their quirk gives them an edge in creating the most beautiful and powerful music, but they face jealousy from others who don't have such a useful quirk.
One way is through self - sacrifice. If the character uses their evil quirk to save a lot of people at great personal cost, it could be a step towards redemption. For example, if their quirk is something like mind - control and they use it to stop a powerful villain who is about to destroy a city, but in the process they lose their own ability to use the quirk. Another way is by showing remorse and actively trying to make up for their past actions. They could start doing volunteer work in the community, using whatever skills they have (even if related to their quirk in a non - evil way) to help others.
One interesting plotline could be Izuku using his voice quirk to communicate with animals. Since many animals respond to different frequencies and tones, his voice quirk might allow him to understand and direct them. For example, he could use it to get a flock of birds to create distractions during a villain attack. Another plotline might involve Izuku using his voice quirk to soothe people in distress. His voice could have a calming effect, which would be useful in rescue situations or when dealing with panicked civilians. And a third plotline could be Izuku having to train his voice quirk in secret because it's considered too dangerous or unpredictable, and he has to find unique ways to control and develop it without getting caught by the wrong people.
One interesting plotline could be Izuku using his Pokemon - like quirk to battle villains in a unique way. For example, he might summon Pokemon - inspired creatures with different powers to counter various villain attacks.
His quirk could be something like super - speed regeneration. In the original Naruto series, he has a great deal of stamina and the ability to heal relatively fast. But with this quirk, he could recover from even the most severe injuries in seconds. This would mean he could keep fighting no matter how badly he was hurt, and his enemies would find it extremely difficult to defeat him.
One idea could be Izuku struggling to control his vampire quirk at first. For example, he might accidentally bite a classmate during a training session when his quirk goes out of control. Another plot could involve Izuku having to hide his vampire nature from his friends and family for fear of being ostracized. Maybe he only reveals it to a select few who help him learn to use it for good.
One interesting story could be where Izuku's portal quirk allows him to access different dimensions filled with unique heroes and villains. He might use this to learn new combat techniques.