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.
Another plotline could involve them traveling to the future. Maybe they see a world where the Jedi have been fully restored and are working in harmony with the galaxy. This could make Vader question his past actions even more, and Luke could gain new insights into the future of the Jedi Order.
A third plotline could be set in an alternate universe where Luke and Leia meet under different circumstances. For example, they could meet as children before they were separated. This would completely change the dynamic of their relationship and lead to a whole new set of adventures and character developments.
Well, one possible plot could be that Darth Vader tries to persuade Luke to join the dark side once again. Maybe he shows Luke some 'truths' about the Jedi that he believes are lies, trying to shatter Luke's faith in the Jedi way.
Sometimes, new characters are introduced in these fanfictions that can be really well - written. They could be characters who act as mediators between Luke and Vader, or they could be side characters who are affected by Luke and Vader's interactions. For instance, a young Jedi apprentice who looks up to Luke and is terrified of Vader, but through the story, comes to see Vader in a different light.
One interesting plotline could be Lorelai and Luke going on a spontaneous road trip together. They might discover new things about each other and their relationship deepens during the journey.
A possible plotline is their family life. In fanfiction, we might see how they raise their children, if any. Maybe they have to protect their family from the dark forces that Vader is often associated with, while also trying to keep their relationship strong.
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 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 possible plotline could be Anakin, in a redeemed state, secretly protecting baby Luke from the remaining threats in the galaxy. He uses his remaining Force abilities to shield Luke from danger without revealing his identity.