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.
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 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.
Harry's view of the world might change completely. He has seen so much darkness and death. With his parents' return, he could start to see the world in a more hopeful light. He may be more willing to forgive those who have wronged him in the past, like Snape. Their return would be a catalyst for a huge shift in his personality, making him more balanced and less haunted by his past traumas.
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 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 great 'Harry Potter Lily and James Fanfiction' is 'All the Young Dudes'. It delves deep into their time at Hogwarts, their relationship development, and how they became the people we know from the main series. It's well - written with great characterizations.
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.
Well, it could lead to a very different story. Harry's childhood and upbringing would be completely changed, and the dynamics of the wizarding world might shift.
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.