One possible plotline could be that James and Lily return and have to deal with the changed wizarding world. They might be shocked at how much Voldemort's actions have affected society even after his defeat. Another could be that they return to help Harry face new threats that are emerging from the shadows of the old war.
A plotline could be that the Potter family gets invited to a very exclusive and mysterious magical event. But there are rumors that dark forces are at work there. Harry and Ginny are hesitant, but James is excited, and Albus and Lily are curious. As they attend, they find themselves in the middle of a conspiracy that they have to solve together.
One possible plotline could be that Lily and James return during the time of Harry's battle with Voldemort. Their return could give Harry new strength and hope. They might use their old magical knowledge and skills to help in the fight. Maybe they could also reunite with their old friends like Sirius and Remus, and together they form a powerful force against the Dark Lord.
There could also be a plotline where their pregnancy is put in danger. Maybe there are dark forces at work in the wizarding world that target them because of their unborn child. This can create a lot of tension and drama, as they have to fight to protect their family. It can also be a way to explore the darker side of the Harry Potter universe and how Lily and James would handle such threats while being pregnant.
One possible plotline could be that Voldemort's attack on the Potter family fails in a different way. Maybe James uses a powerful, unknown spell that deflects Voldemort's killing curse. As a result, Lily and James go into hiding with Harry. They might then start to form an underground resistance against Voldemort, using their knowledge of the Dark Lord's weaknesses. This could lead to a completely different wizarding war, with the Potters at the forefront, and Harry growing up with his parents by his side, learning magic from them directly.
One idea could be that they return with new magical abilities. Maybe James has the power to control time briefly, which he uses to prevent some small tragedies in the wizarding world. Another could be that they return to find that Harry has changed the wizarding world in ways they never expected, and they have to adapt to this new world order.
One interesting fanfiction could be about James and Lily's time at Hogwarts before they got together. It might explore how their relationship developed from initial dislike to love. For example, a story could show James maturing from a prank - loving boy to someone worthy of Lily's love.
One common plotline is time - travel. For example, Harry might go back in time and interact with Lily Potter in a way that changes the course of events. Another could be an alternate universe where Lily survives and she and Harry have a different kind of relationship, perhaps more of a mentoring one.
Harry would likely be overjoyed at first. After all, he never really got to know his parents. Their return could give him a sense of family and belonging that he's always craved. It could also help him deal with some of his past traumas, having his parents there to support him.
One interesting plotline is when Lily is given the chance to change the future. In some fanfictions, she has the power to go back in time and make different choices, like whether to marry James or not. Another is when she discovers hidden magic within herself that no one else knew about. This often leads to her having to face new challenges and enemies.
Obviously, James and Lily Potter are the main characters as the title implies. Their return is the central event. Harry Potter is also a key character as their return would greatly impact his life.