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.
Another interesting fanfic idea is to create an OC who is a rebel spy that manages to infiltrate Vader's inner circle. This OC could be constantly trying to gather information about the Empire's next moves, all while trying to avoid being detected by Vader, who is always one step ahead. There could be a lot of tense moments as the OC tries to outwit Vader.
A third idea is to have an OC who is a Jedi survivor from Order 66. This OC, believing Vader was once Anakin Skywalker, tries to bring him back to the Light Side. The fanfic could explore their interactions, with the OC using memories of Anakin's past to try and break through Vader's Sith persona.
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.
One plot idea could be a time - travel scenario where Ahsoka is sent back in time and has to face Darth Vader before he fully became the Sith Lord she knew. She could try to prevent his fall. Another idea is a story set in an alternate universe where Ahsoka and Vader team up against a greater threat, forcing them to put aside their differences. Maybe a plot where Ahsoka is captured by Vader and he has an internal struggle about what to do with her, given their past relationship.
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.
Well, you could have a fanfic where Darth Vader travels back in time to the Clone Wars era. He could try to change certain events to better suit his later goals as a Sith. For example, he might attempt to influence the development of the clone army in a way that would make them more loyal to him personally rather than the Jedi or the Republic. This could lead to all sorts of conflicts and power struggles within the already complex Clone Wars setting.
A plot could involve Ahsoka trying to redeem Darth Vader. She might attempt to reach out to the Anakin Skywalker she once knew, and through a series of encounters, Vader starts to question his path in the dark side. For example, they could be forced to team up against a common enemy, like a new Sith Lord who wants to destroy them both. This situation would put Vader in a position where he has to choose between the Sith teachings and his old friendship with Ahsoka.
One idea could be a what - if scenario where Anakin in his pre - Vader state somehow gets a glimpse of his future as Vader. He could then fight against the dark side within himself that would turn him into Vader. Another could be a time - travel plot where Vader is sent back to face Anakin, and Anakin has to decide whether to accept his possible future or fight against it.
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.
There's the character of Lyra, an OC in some fanfics who is a mechanic on the Death Star. She has a run - in with Vader and through a series of events, starts to question the Empire. She's popular because of her plucky attitude in the face of such a menacing figure as Vader.
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.
One idea could be a crossover where Darth Vader somehow ends up in the wizarding world. Maybe he's mistaken for a very powerful dark wizard and Hogwarts has to figure out how to deal with him. Another could be a story where Harry Potter discovers the Force and has to learn to use it alongside his magic. A third idea might be a scenario where Darth Vader is resurrected in the Potterverse and teams up with Voldemort, causing chaos for the Order of the Phoenix.