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.
Maybe the male OC is an exchange student from another country with a very rare quirk. His foreign charm combined with his quirk, say something like the ability to communicate with animals, draws in a diverse harem. There could be a girl who is interested in animals and wants to learn more from him, another who is simply fascinated by his different cultural background. As they interact more, they form a close - knit group that supports each other in their hero - training journey.
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.
Maybe the male OC is from another dimension and gets stranded in the Teen Titans' world. His different cultural background and knowledge from his own dimension intrigue the Titans. The harem dynamic could start with simple friendships that gradually turn into something more. Raven might be attracted to his calm and collected nature in the face of the strange new world, while Starfire could be interested in his stories of his home dimension. Beast Boy might see him as a cool new friend at first and then develop deeper feelings as they spend more time together.
One idea could be to make the character with the lightning quirk have a fear of water. This would add an interesting dynamic as lightning and water don't mix well. They could struggle to control their quirk when near water, and it could be a source of internal conflict for them.
The male OC could be designed with a laid - back personality but with a hidden intensity when it comes to using his quirk. Physically, he could have longish hair that he often brushes out of his face in a cool way. He might wear a combination of modern streetwear and some traditional hero - like accessories, like a cape - like scarf. His quirk could be related to energy projection, which gives him an edgy look when he uses it, with glowing energy coming from his hands.
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.
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.
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.
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.