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.
A plot could revolve around the little angel being a key to a long - forgotten Jedi power. Luke, with his connection to Vader, has to figure out how to unlock this power within the little angel without falling to the dark side himself. The little angel could be a test of his Jedi resolve and his ability to redeem Vader.
The little angel could start off as a mysterious and seemingly innocent being. As the story progresses, it could be revealed that it has a much deeper knowledge of the Force and the history of the Skywalker family. For example, it might know secrets about Anakin Skywalker's fall that no one else does.
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.
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.
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.
Here are some plot ideas. Imagine a fanfic where Padame is resurrected by an ancient Force power. She then disguises herself and infiltrates the Empire to be close to Anakin. As she gets closer to him, she starts to bring back his memories of their love. For Luke and Leia, they could be on a quest to find a powerful Jedi artifact that was once owned by their mother. Along the way, they face many challenges and have to use their combined skills. Another idea is a fanfic set in an alternate universe where Anakin never turned to the dark side. He, Padame, Luke, and Leia all live together as a happy family, and they are on a mission to spread peace and justice across the galaxy.
One possible plot could be that Luke discovers he has a special connection with Vader that goes beyond their typical relationship in the Star Wars universe. Maybe he starts having dreams where Vader is trying to teach him a new form of the Force, something that was lost to the Jedi. And as Luke delves deeper into these dreams, he begins to understand Vader in a more empathetic way.
Well, it could be a fanfiction where Luke and Vader have an unexpected relationship, perhaps a more tender or different take on their usual dynamic, like Vader showing a soft side towards Luke as if he were a little baby in terms of being protective or having some newfound paternal feelings. But without reading it, it's hard to say exactly.
Another plotline might be that Vader has a secret mission that requires Luke's help. They have to team up despite their differences, and along the way, they start to understand each other better. For example, Vader could be trying to stop a greater evil in the galaxy that even the Emperor doesn't know about.
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.