One common plot idea is the OC trying to become the top hero. They use their knowledge from their past life to gain an edge. For example, they know the training methods of the best heroes and apply them to their own training. Another idea is the OC changing the fate of certain characters. Maybe in the original story, a side character dies, but the OC intervenes to save them. Or they could be involved in uncovering a conspiracy within the hero society that was hidden in the original story.
A popular plot could be the OC struggling to fit in the hero world at first. They might have different values from the native characters because of their past life. But as they learn more about the world, they start to make their mark. Also, there could be a plot where the OC discovers a new type of quirk that changes the balance of power in the hero world. This could lead to them being both feared and respected. Additionally, the OC could form a team with other misfits and together they rise through the ranks of the hero world.
A more complex plot could be that the OC reincarnates into a world where the events of 'boku no hero academia' have taken a different turn. Maybe All Might was defeated much earlier and the villains are in almost complete control. The OC has to find a way to gather the remaining heroes, train them in new ways, and inspire hope again. This would involve a lot of character development as the OC has to prove themselves to the jaded heroes and also face the very powerful villains.
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.
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 idea could be a female OC who has a unique quirk related to sound manipulation. She could use it to both heal and attack. For example, she might be able to use certain frequencies to soothe injured heroes or use ultrasonic waves as a weapon against villains. Another plot could involve a female OC from a foreign country coming to study at U.A. High School. She has to deal with cultural differences while also trying to fit in with the other students and prove herself as a hero. A third idea is a female OC who discovers an ancient quirk that has been lost for generations and has to learn how to control it while also facing those who want to steal her new - found power.
A cool plot for a male oc fanfiction could be an oc who is from a family of villains but wants to be a hero. He has to constantly fight against the stigma and suspicion from his classmates and teachers. He might have a quirk that is seen as villainous, like the power to create illusions that can cause fear, but he uses it in positive ways, like creating distractions during rescue missions. Also, an oc with a quirk that lets him transform into different elements could be interesting. He could struggle with controlling his powers at first and accidentally turn into water during a classroom session.
The OC could have a quirk that gives them the ability to see all possible futures. This would make their decision - making in battles very crucial. They might struggle with the weight of this knowledge at first, but as they learn to trust their instincts and use their quirk effectively, they become an invaluable asset to their hero team. They could also use this quirk to prevent major disasters in the My Hero Academia world.
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.