Here are some plot ideas. The Saiyan OC could be a transfer student from a far - off planet to U.A. High. They have to adjust to the school's rules and the different way of using powers. There could be a tournament arc where the Saiyan's unique powers shock everyone. Another plot could involve a villain who wants to steal the Saiyan's power source. The Saiyan has to protect it while also not causing too much destruction in the process. Also, a plot where the Saiyan tries to teach the other students some of their Saiyan training techniques to help them level up their quirks.
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.
First, think about their Saiyan powers and how they would translate in the My Hero Academia world. For example, a Saiyan's ability to increase power level could be like a quirk that gets stronger over time. Consider their backstory, like maybe they're a Saiyan from another planet who ends up in the MHA world. Also, their appearance can combine Saiyan features like a tail and spiky hair with the MHA style of clothing.
One interesting concept could be the combination of Saiyan powers with the unique quirks in My Hero Academia. For example, a Saiyan - like character might have a quirk that is enhanced by their Saiyan heritage, such as super strength getting an extra boost from Saiyan battle rage. Another concept could be the cultural clash between the Saiyan warrior culture and the hero society in My Hero Academia. It would be fascinating to see how a Saiyan - raised character adapts to the rules and values of the hero world.
Often, Izuku Midoriya is a popular character in these fanfictions. Writers like to give him Saiyan powers and explore how his original quirkless state would be affected. Bakugo Katsuki is also common. His aggressive nature can be easily paired with the Saiyan's warrior spirit. And All Might might be included as well. His status as a symbol of peace could be interesting when combined with Saiyan strength, like he could be an even more powerful mentor to those with Saiyan - influenced quirks.
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 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.
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.
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.
Perhaps the OC has a hidden past related to a villain organization, and the harem members are initially suspicious but as they get to know the OC better, they realize there's more to the story. This could create a lot of drama and conflict as the OC tries to prove their loyalty while also dealing with their growing feelings for the harem members.