One plot idea could be that your OC is wrongly accused of being a villain because their methods of killing villains are too extreme for the hero society. They then have to prove their innocence while still continuing to hunt down the real villains. Another idea is that a powerful villain organization puts a bounty on your OC's head, and they have to fight off assassins while also taking on the main villains.
One plot idea could be a villain who is secretly a double agent, working for the heroes all along. They infiltrate the villain group and gather information. Another idea is a hero who loses their quirk and has to rely on their wits and combat skills alone, like Batman in the DC universe. And a villain - hero team - up to face a greater threat that neither can handle alone.
Well, a 'My Hero Academia Villain Killer' fanfiction can be really exciting. It often explores the darker side of the MHA universe. Some fanfictions in this genre might focus on a new character who has a vendetta against villains. They could introduce unique powers and moral dilemmas. For example, the character might struggle with the line between justice and revenge while taking on the most heinous villains in the series. It adds a whole new layer to the already rich MHA world.
One plot idea could be that the Saiyan OC is mistaken for a new type of villain at first. As they try to prove they're not a threat, they start to make friends and learn about heroism. Another idea is that they discover a connection between Saiyan powers and some of the hidden secrets in the MHA world. Maybe there's an ancient power source that only a Saiyan can unlock.
A great plot could involve the ruthless OC being a double agent. They pretend to be a hero but are actually working for a secret villain group. As they interact with the heroes, they start to have doubts about their loyalty to the villains. Maybe they fall in love with a hero, which further complicates things. Another plot is that the OC has a vendetta against a particular hero or villain family. They use their ruthless tactics to systematically take down members of that family one by one. Also, the OC could be part of an experiment that gives them super - ruthlessness, and they have to deal with the consequences of this new - found trait.
One idea could be an OC with a unique quirk that doesn't seem powerful at first but has a hidden potential. For example, an OC who can manipulate shadows in a very limited way, but discovers they can use it to travel between dimensions in the shadows. Another could be an OC who is a foreign exchange student with a quirk that is considered abnormal in the My Hero Academia world, like a quirk that allows them to communicate with plants and use them as allies in battle. Or an OC who has a quirk that lets them see the future but only in their dreams, and they have to figure out how to use this information to help their classmates and heroes.
Well, for a great villain killer OC in My Hero Academia fanfic, think about their motivation deeply. It could be revenge for a lost loved one due to a villain attack. In terms of appearance, make them stand out. For example, they could have a scar across their face from a past encounter with a villain. Their fighting style should be interesting too. Maybe they use a combination of martial arts and their quirk in a very unorthodox way that catches villains off guard.
One idea could be that Deku gets tired of being constantly underestimated and bullied despite his efforts to be a hero. So, he decides to use his knowledge of hero strategies against them and becomes a villain. He could form his own group of outcasts who were also treated unfairly in the hero - centered society.
Another plot could involve Ochako being manipulated by a charismatic villain. This villain convinces her that the hero society is corrupt and that by joining the 'dark side' she can actually make a more positive change. She then uses her gravity quirk to disrupt hero operations, like making heroes' equipment float away during battles.
One idea could be that the 'Villain Class 1A' has to team up with the real Class 1A to fight a greater threat. Maybe a new super - villain emerges who is too powerful for either group alone. This would lead to some really interesting character dynamics as they have to put aside their differences.
One popular archetype is the power - hungry villain. They crave more and more power, like All For One in the original series. Another is the vengeful villain. They have a grudge against someone or the whole hero society and will stop at nothing to get their revenge. There's also the mad scientist type. They conduct unethical experiments, often creating dangerous things in the pursuit of knowledge or power.