Well, a really cool plot for Luke Skywalker and Darth Vader fanfiction could be that Vader starts having visions of a different future where he never turned to the Dark Side and he tries to change his present self with Luke's help. Or perhaps a story where they are both sent back in time to the days before Anakin's fall, and they have to figure out how to prevent the rise of the Empire without changing too much of the past. It could also be interesting if in a fanfic, Luke is somehow forced to become a Sith apprentice to Vader to save the lives of his friends, but he tries to turn Vader back to the Light Side from within the Sith ranks.
For Luke Skywalker and Darth Vader fanfiction, a great plot could be set in an alternate universe where Vader is actually a double agent for the Rebellion all along. Luke discovers this truth accidentally and they have to work together more closely to bring down the real villains in the Empire. Another plot could involve a magical or Force - related artifact that can only be activated by both of them together. They must put aside their differences and combine their powers to use this artifact for the greater good of the galaxy. And yet another idea is a fanfic where Vader, after a near - death experience, has a moment of clarity and asks Luke to train him to become a Jedi again, which would be a huge shift in their relationship and a very challenging journey for both of them.
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 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.
A great plot idea could be that Darth Vader is ordered by the Emperor to kill Baby Luke, but he can't bring himself to do it. Instead, he fakes the baby's death and takes him to a hidden Jedi enclave. There, with the help of some remaining Jedi, they start to train Luke in secret. Meanwhile, Vader has to constantly deceive the Emperor about Luke's whereabouts and his own actions regarding the boy. This would create a lot of tension and drama in the fanfiction as Vader is walking a very dangerous line between the dark side and a newfound sense of hope through the baby.
Perhaps they time travel to a parallel universe. In this universe, Darth Vader never turned to the dark side and Anakin Skywalker remained a Jedi. Luke would be able to see how different things could have been. This experience might inspire Vader in their own universe to try and find a way back to the light side, and they could then work together to bring positive change to their own timeline.
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.
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 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 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.
A young, Force - sensitive alien who has the power to manipulate time could be an interesting addition. This character could bring a new dynamic to the relationship between Luke and Vader. For example, they might have to protect this alien from the Empire or the Sith who want to use its power for evil.