Inspiration can also come from other works of fiction. Maybe there's a movie or a book with a cool concept that you can adapt to the MHA world. For instance, a time - travel concept could be used to send your OC back in time to change a crucial event in the MHA history. And don't forget to explore the different locations in the MHA world. You could set a part of your story in an unexplored area, like a hidden villain lair deep in the mountains, which your OC has to infiltrate.
Another source of inspiration is real - life events. If there's a social issue like discrimination, you can translate that into the MHA universe. Suppose your OC is from a group that's discriminated against because of their quirk type. This could lead to a story about fighting for equality. Also, think about the relationships between characters in the show. What if your OC has a complicated relationship with a canon character? That could be a great plot point.
An interesting plot could involve an OC who is initially thought to be quirkless but discovers a latent quirk later on. This OC could then struggle to catch up with the others in terms of training and development. Or, an OC with a shapeshifting quirk could be used in a plot where they have to infiltrate a villain's lair by taking on the form of one of the villains' minions. They would have to be careful not to be discovered and also gather intelligence along the way.
Well, he could be an OC with a healing quirk. In the fanfiction, he starts helping out injured students during training sessions. This catches the attention of several female characters. Some might see him as a valuable asset to their future hero teams, while others develop a more personal attraction. The harem could then be formed as these girls start competing for his attention in a friendly way, like trying to bring him the best lunches or inviting him to exclusive study sessions.
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 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.
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.
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.
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 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.
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.