One plot idea could be that the OC criminal is secretly working with a high - ranking hero to bring down a powerful villain organization from the inside. Another is that the OC criminal has a personal vendetta against a specific hero and is constantly trying to undermine them. A third idea is that the OC criminal discovers a powerful but dangerous quirk - enhancing drug and is trying to distribute it among the criminal underworld.
A great plot idea could be the OC criminal is actually an experiment gone wrong by a shadowy government agency. They're now on a mission to expose the agency's unethical practices while also causing chaos as a criminal. Or perhaps the OC criminal has a unique ability that allows them to steal other people's quirks temporarily, and they're using this to build an army of sorts. Another plot could be that the OC criminal is trying to find a way to cross into another dimension where there are no heroes to stop them.
One plot idea could be your OC starting at a new hero school. They have to deal with fitting in, making friends, and facing rivals. Another is that your OC discovers a new type of quirk that is being hunted by villains. They then have to protect this knowledge while also learning to control their own quirk better. Or, your OC could be wrongly accused of a crime committed by a villain with a similar quirk. They have to clear their name while also dealing with the public's distrust.
An interesting concept would be an OC who has a quirk that is based on art. For example, they can bring paintings to life. This OC could be a part of an art school within the My Hero Academia universe and use their quirk to fight villains who are attacking art galleries or museums. They would also have to train their quirk to be more combat - effective.
A good plot for My Hero Academia Vigilante OC fanfiction could start with the OC finding a strange quirk - enhancing drug on the streets. This leads them to investigate a new criminal organization that is creating and distributing these drugs. As they dig deeper, they realize that this organization has ties to some powerful people in the hero world. Along the way, they might meet an old friend who has turned to the dark side and is involved with the organization. They have to decide whether to save their friend or focus on taking down the organization. Another plot could be that the OC's secret identity is at risk of being exposed when a nosy journalist starts following their every move.
A good plot could involve the assassin OC having a past connection with a major villain in the My Hero Academia world. They are forced to confront this villain when the villain discovers their existence and tries to either recruit or eliminate them. Or perhaps the OC is on a mission to steal a powerful quirk - enhancing drug, but the heroes get in their way and they have to decide whether to fight or cooperate.
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.
One great plot idea is having your SI OC be a transfer student from a foreign hero school. They bring in new techniques and a different perspective on heroics. Another idea could be that your OC has a quirk that is initially seen as a villain's quirk, and they have to prove that it can be used for good. For example, a quirk that allows them to control minds but they use it only for interrogation purposes in hero work.
Your fanfiction could start with the OC accidentally stumbling upon Todoroki in a moment of vulnerability. Maybe he's having a flashback about his father. The OC offers comfort, and this starts their relationship. Then, they could be chosen to represent their class in a hero tournament. Working side by side, they face tough opponents and their bond grows stronger. You could also add a sub - plot where other students are jealous of their newfound closeness, creating some interesting drama.
To create a great My Hero Academia OC criminal, focus on their appearance. Dress them in a way that reflects their criminal nature, like dark colors and a menacing look. Also, consider their relationships. They might have a group of other villains they work with or have a nemesis among the heroes. Their personality should be complex. They're not just evil for the sake of it. Maybe they have a code of ethics, even if it's a twisted one.