A great plot could be that the male OC has a past connection to one of the main Avengers villains. For example, he was experimented on by the same organization that created the Hulk. This leads to a complex situation where the Avengers don't know if they can trust him. Or, the male OC could accidentally time - travel and change a small event in the past that has major consequences for the present - day Avengers, and he has to fix it.
One plot idea could be that the male OC discovers a hidden vampire society within Mystic Falls that has been kept secret from the main characters. He then has to decide whether to reveal it to them or keep it a secret for his own gain. Another idea is that he falls in love with a human girl, but she is being hunted by a powerful vampire hunter. He has to protect her while also hiding his true nature from her.
A good plot could involve the male OC finding a cure for the walker virus (even if it's just a false lead at first). He becomes obsessed with it, and this drives his actions and relationships. Or perhaps he is wrongly accused of stealing from his group, and he has to prove his innocence while also dealing with the external threats of the apocalypse.
One plot idea could be that the male demon OC is sent to disrupt the quest of the hobbits. He tries to steal the One Ring for his own dark purposes. Another idea is that he forms an unlikely alliance with the hobbits. Maybe he realizes that by helping them, he can gain something in return, like a way to redeem himself. Or perhaps he is in search of a lost relic in Middle - earth and his path crosses with the hobbits, and they end up having to cooperate to find it.
For an Inuyasha male OC fanfiction plot, consider this. The male OC is an artist who can paint things that come to life. He gets caught up in the chaos around the Shikon Jewel when one of his paintings accidentally becomes a powerful demon. Now he has to work with Inuyasha and the others to defeat it. Or he could be a demon slayer who has a personal vendetta against a particular type of demon that is also involved in the Shikon Jewel saga. His presence disrupts the group's dynamics at first, but they soon realize they need his skills. Another idea is that he's a messenger from another dimension who brings news of a greater threat that will affect not only the Inuyasha world but also other dimensions.
A male OC could be a Muggle - born who discovers he has a special power that the wizarding world has never seen before. He could be hunted by dark forces who want to use his power for evil, and he has to rely on his new friends at Hogwarts to protect him. Another idea is that the male OC is a transfer student from a foreign wizarding school. He brings with him different magical techniques and cultural beliefs, which causes a stir at Hogwarts as students and teachers alike are both intrigued and a bit skeptical.
One plot idea could be that the male OC is a new student at True Cross Academy. He's a bit of an outcast because of his mysterious past. As he tries to fit in, he discovers a hidden plot within the academy that involves demons. Another idea is that he has a special power that the exorcist order wants to use for their own purposes, but he has to decide whether to cooperate or not. A third plot could be that he is on a quest to find a lost artifact that has the power to either destroy all demons or give them ultimate power, and he has to face many challenges along the way.
Here's a plot idea. The male OC is an alchemist who has a special connection to the Truth. He starts having visions of a coming catastrophe that is related to the misuse of alchemy. He has to convince the other alchemists, including Edward and Alphonse, that this is real and they need to work together to prevent it. This could involve him traveling across the country, facing various challenges and enemies along the way. Another plot could be that he's a foreign alchemist who comes to Amestris to study their alchemy. But he soon realizes that there are dark secrets within the Amestrian alchemical community that he must expose.
Your OC could be a ranger who has a vendetta against a powerful orc chieftain. The orc has been raiding villages and killing innocents. The OC sets out on a mission to hunt down the orc. Along the way, they get involved in a larger conflict between the forces of good and evil. They meet other characters from the Lord of the Rings world, like Aragorn or Legolas. This plot allows for a lot of action and character development as the OC battles their personal enemy while also contributing to the greater fight against Sauron.
A good plot could involve the male elf OC being sent on a mission by his elven king to spy on the dwarves' journey. But during this, he realizes that the dwarves are not as bad as he thought and decides to help them instead. He could also be cursed by an evil sorcerer, and the only way to break the curse is to complete a series of impossible tasks. This could lead him to team up with unlikely allies, like a hobbit or a wizard. Additionally, his family's past could come back to haunt him, and he has to face his ancestral demons to save his people.
One plot idea could be that the male OC is a detective hired to investigate strange disappearances in the Black Butler world. He starts to suspect that there's a supernatural force at work and teams up with the main characters to solve the mystery. Another idea is that he's a nobleman's son who has a secret identity as a vigilante, fighting against the corrupt nobles. He might cross paths with Ciel and Sebastian when they're on a mission related to the same nobles.