For plot ideas, think about her past. Maybe she made a mistake centuries ago that is now coming back to haunt her. She could be hunted by an ancient organization that wants to use her immortality for their own ends. Another idea is that she falls in love with a mortal hero, and they have to deal with the implications of her immortality on their relationship. And she could also be involved in a multi - dimensional crisis, using her immortality - related knowledge to solve the problem.
One plot idea could be that she has to stop a villain who is trying to steal her immortality. Another is that she discovers a new threat to the world that only she, with her long - lived experience, can understand. Maybe she teams up with a group of heroes who initially don't trust her because of her immortality.
One plot idea could be that the OC female mercenary is hired to steal a valuable artifact in the Mass Effect universe. But as she's on the mission, she discovers that the artifact has more power and significance than she thought, and she has to decide whether to complete the job or do the right thing. Another idea is that she gets caught up in a conflict between different alien races and has to choose a side.
One plot idea could be that the female OC is on a quest to find a hidden treasure that was once her family's. Along the way, she has to outwit other pirates who are also after the treasure. Another idea is that she is wrongly accused of a crime she didn't commit in a pirate port and has to clear her name while also dealing with the various pirates and scoundrels in the area.
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.
One plot idea could be that the OC fem synth discovers she has a hidden ability that the Institute didn't even know about. Maybe she can communicate with long - lost pre - War technology. This leads to a race between different factions in the Fallout world to either control her or destroy her. Another idea is that she falls in love with a human, which is against the Institute's rules. This causes a lot of internal conflict for her as she tries to balance her feelings with her programming.
One idea could be that the immortal female OC has lived through different historical periods and uses her knowledge to solve modern-day mysteries. For example, she might have witnessed the fall of ancient civilizations and can now help archaeologists understand things that were lost to time. Another plot could involve her falling in love with a mortal and having to deal with the heartbreak that comes with outliving her lover over and over again. Or perhaps she is on a quest to find others like her, immortals who have been hiding in the world for centuries, and along the way she discovers secrets about the origin of her immortality.
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 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.
A good plot could involve a human OC who is a doctor. They end up treating a ghoul who was severely injured in a fight. This starts a strange relationship where they try to understand the nature of ghouls better. As they get more involved, they learn about a secret organization within the ghoul world that is planning something big that could endanger all of Tokyo. The OC then has to decide whether to stay out of it or try to stop it. It's a plot full of moral dilemmas and action. Also, a plot where the human OC is an artist and they use their art to show the hidden side of the ghoul - human relationship could be interesting. Their art could either be a way to promote peace or it could stir up more trouble depending on how different groups react to it.
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.