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.
One plot idea could be an OC who is secretly searching for a way to reverse the effects of human transmutation without the use of the Philosopher's Stone. They could be an outcast in the alchemist community because of their unorthodox methods. Another idea is an OC who discovers a hidden alchemical society that has been operating in the shadows, with their own set of rules and secrets that could change the balance of power in the Fullmetal Alchemist world. A third plot idea could be an OC who has the ability to see the souls of those who have been affected by alchemy, and they use this ability to solve mysteries related to past alchemical experiments.
The plot could involve the OC being a alchemist on a quest for a powerful alchemy item. Greed, sensing an opportunity for himself, joins the OC. Along the way, they encounter various enemies and obstacles. For instance, they might have to face off against other homunculi who don't want Greed interfering in their plans. The relationship between Greed and the OC could start off as a simple partnership but evolve into a deep friendship or more as they rely on each other to survive.
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.
One idea could be an OC who is a foreign alchemist with a unique form of alchemy based on their homeland's culture. For example, an alchemist from a desert region who uses sand-based alchemy. Another could be an OC that has a special connection to the Homunculi, not as an enemy but as someone trying to understand and redeem them. Maybe an OC that was experimented on by the same people who created the Homunculi and has some shared traits.
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.
Well, here's a plot idea. The male OC has a special connection to a magical artifact that is coveted by an evil force. He has to protect it while also trying to figure out why he has this connection. Also, he could be in a love triangle with two other characters in the guild, which adds a lot of drama to the story. Another idea is that he's the key to unlocking a hidden power within the Fairytail guild, but he doesn't know it at first and has to go through a series of trials to discover his true potential.
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.
A plot could involve the male OC being related to one of the scientists who worked at the lab. He comes to Hawkins to find out what really happened to his relative. Along the way, he teams up with the kids and they uncover a conspiracy that goes deeper than they thought. Or perhaps the male OC is an artist and he starts having visions of the Upside Down through his art, which leads him on a journey to figure out what they mean.
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.