One possible plot could be that the OC is a new mutant who arrives at the X - Men school. Jean Grey senses a powerful and unique energy within the OC. Scott is initially cautious but as they all train together, the OC starts to develop feelings for both Jean and Scott. Meanwhile, a common enemy emerges that forces them to work closely, and during this time, a complex love triangle forms as Jean and Scott also find themselves drawn to the OC in different ways.
Mr. Sinister could be a great antagonist. He might be interested in the OC's unique genetic makeup, which could be related to Jean and Scott's powers in some way. He could try to manipulate the situation to his advantage, perhaps by pitting Jean and Scott against each other over the OC, or by trying to turn the OC against them.
It's a bit complicated. Jean Grey's fate varies depending on the comic storyline and continuity. Sometimes she's dead, but often there are plot devices that bring her back.
One common theme is friendship. Since they both have powers, they often form a strong bond in fanfiction. They support each other and learn from one another.
One common theme is self - discovery. The OC often has to figure out their powers and place in the world of mutants, similar to Scott's own journey. Another theme is friendship. They form bonds with Scott and other X - Men members. For example, in many fanfictions, the OC starts as an outsider but gradually becomes an integral part of the team.
Another alternative is to find fanfiction that delves into his family relationships. For instance, his relationship with his brother, or how his family history has shaped him as a person. These types of stories can provide a lot of depth to the character without involving sexual content.
One of the best Jean Grey stories is 'The Dark Phoenix Saga'. It's a classic that shows Jean's struggle with the powerful and dangerous Phoenix Force. Her transformation into the Dark Phoenix and the resulting chaos is both thrilling and tragic.
Love and loyalty are common themes. Usually, the reader and Scott develop a deep love for each other, and they show great loyalty in the face of difficulties. For example, in many stories, when Scott is in trouble because of his mutant identity, the reader stays by his side.
Power dynamics can be a theme. If the OC has powers, how those powers interact with Scott's powers is explored. Family is another theme sometimes. The OC might come from a different family situation and how Scott and his mutant family react to that is part of the story. And of course, adventure is a common theme as they go through various challenges together.