It could be that their return is tied to some ancient magic. They come back with a mission to correct the wrongs in the wizarding world that led to Voldemort's rise in the first place. They might work on uniting the pure - bloods, half - bloods, and Muggle - borns in a more profound way, using their own relationship (Lily being Muggle - born and James a pure - blood) as an example of how harmony can exist.
Perhaps they return in a sort of spirit form at first. They could start by guiding Harry in his dreams, passing on important information about Voldemort's weaknesses. As the story progresses, they find a way to fully return to the physical world. They could then start to rally the wizarding community in a different way than Dumbledore did, bringing a fresh perspective to the fight against the Dark Arts.
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.
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.
One possible plotline could be a time - travel scenario. Maybe Ginny somehow travels back in time and meets James and Lily. They could form a bond and Ginny could learn a lot about Harry's early life and family history directly from them.
Well, a plotline might involve them facing challenges from the wizarding world. For example, there could be threats from dark wizards which they have to fight together as a married couple. Or perhaps they get involved in some sort of magical research as a team in their married life. Also, their relationship growth is a big part. They could have little arguments and then make up in the most romantic ways, like James bringing Lily her favorite flowers after a fight.
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.
Well, one plotline could be that Lily uses her influence to change the way Hogwarts is run. Since she was a brilliant witch, she might introduce new spells and teaching methods. For example, she could start a special class for students to learn advanced defensive magic earlier.
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.
One possible plotline could be that Lily, now as a girl, has to navigate the wizarding world all over again. She might face different challenges and opportunities than she did before. For example, she could form new friendships or rivalries at Hogwarts.
They could have adventures exploring different realms of heaven. Maybe there are places full of magical creatures they've never seen before. They might be on a quest to find a special kind of magic that can help Harry in some way, even from the afterlife.
One great fanfiction could be 'Lily and James: A Love Beyond'. It delves deep into their relationship before Harry was born. It shows how they met at Hogwarts, their initial dislike which gradually turned into love. The story also includes their adventures as members of the Order of the Phoenix and how they fought against the Dark forces side by side.
One possible fanfiction could focus on the moments leading up to their death. It might show James trying to protect Lily and Harry until the very last moment. He could be shown using his wand to fend off Voldemort as long as he could, while Lily was huddled in a corner with baby Harry, praying for a miracle.