Another idea is that the son has a prophetic vision about Vader's future. This causes Vader to question his path in the dark side. The two of them then go on a journey to try and change Vader's fate, which would be a great exploration of their relationship and the power of the Force.
The son could be a healer in the Force, and Vader is initially skeptical. But as the story progresses, he sees how his son's power can be used for good. This leads to a change in Vader's view of the Force and his own actions, and they work together to help those in need in the galaxy.
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.
There could be a fanfiction where Vader's son is kidnapped, and Vader has to go on a quest to rescue him. Along the way, he reflects on his own parenting and how he wants to be a better father. This would add an interesting dynamic to their relationship in the story.
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 Luke discovers a young Force - sensitive 'little angel' figure who has a connection to Vader's past. Maybe this little angel was a symbol of hope on Vader's home planet long ago. Luke then tries to protect the little angel from the remnants of the Empire who want to use its power for evil.
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 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.