The OC could be a mechanic on a Star Destroyer. Darth Vader notices her skills and enlists her help for a special project. As they work together, they discover they have more in common than they thought. The OC begins to see the man behind the mask, and Vader starts to question his loyalty to the Emperor because of her influence.
One idea could be that the OC is a rebel spy who gets captured by Darth Vader. Instead of killing her immediately, he becomes intrigued by her tenacity and they start to have a series of tense conversations that gradually lead to an unexpected understanding between them.
Another plot could be set in an alternate universe where Ahsoka manages to reach out to the little bit of Anakin still left in Darth Vader. She tries to redeem him, and through a series of encounters, they start to question their own paths and the Sith vs. Jedi ideology.
One idea could be a fanfic where Darth Vader has a secret apprentice who is an original character. This OC could be a young Force - sensitive being from a hidden planet. The story could revolve around Vader training this apprentice in the ways of the Dark Side, while also facing internal conflict as he sees some of his own past self in this young OC.
One idea could be exploring Darth Vader's identity before he became encased in the suit. Maybe he rediscovers his old self and tries to reconnect with his past relationships. Another could be him having to face the Jedi without the intimidating suit, relying solely on his raw Force abilities.
Perhaps Darth Vader's dark side powers are misinterpreted in the Naruto world as a new and extremely dangerous form of jutsu. Naruto and his friends might try to find a way to seal these powers. Meanwhile, Vader is on a quest to find a way back to his own universe, and in the process, he starts to question his own dark ways after seeing the camaraderie among the Naruto characters. He could end up having a change of heart and use his powers for good in the Naruto universe.
One idea could be a story where Darth Vader discovers Leia's true identity earlier and struggles with his loyalty to the Empire and his newfound knowledge of his daughter. He could secretly protect her from the Emperor's wrath.
One interesting Darth Vader OC fanfiction could be about a new character who is a force - sensitive mechanic. This character gets caught up in Vader's plans when Vader discovers their unique skills. They might end up helping Vader repair his suit in a more advanced way, leading to some unforeseen consequences and an interesting power dynamic between them.
One idea could be a story where Darth Vader secretly trains Luke in the ways of the Force before Luke fully realizes who he is. This would add an interesting layer of secrecy and moral conflict for Vader.
One idea could be a story where Luke discovers a way to temporarily reverse Vader's Sith corruption, leading to some heart - to - heart conversations between them as Anakin. Another could be a fanfic where they have to team up against a new, greater threat in the galaxy that neither the Rebels nor the Empire can handle alone. Maybe a story where Luke gets trapped in a Sith - created alternate dimension and Vader has to go in and rescue him to atone for his sins.
One idea could be that Darth Vader discovers his connection to Leia earlier and secretly tries to protect her from the Emperor's more sinister plans. He might use his position within the Empire to subtly divert threats away from her.
One idea could be a crossover with Star Trek. Darth Vader could find himself on the Enterprise and have to deal with the Federation's values, which are very different from the Sith's. Maybe he'd be forced to team up with Kirk and Spock to fight a common enemy.