Well, perhaps they survive the attack and then start a sort of underground resistance movement. They gather other witches and wizards who oppose Voldemort and train them in special magic that can combat the Dark Arts. And also, they could be on a quest to find a way to permanently defeat Voldemort by searching for ancient spells or magical artifacts that were long forgotten.
A possible plot is that they are alive but amnesiac after the attack. They end up in a muggle world for a while, slowly regaining their memories. Meanwhile, the wizarding world believes they are dead. As they regain their memories, they start to realize they have a son and that Voldemort is still a threat. They then make their way back to the wizarding world, using their new - found muggle knowledge and skills to their advantage against Voldemort.
One plot idea could be that James and Lily go into hiding with Harry instead of being killed. They could raise him in a secret location, teaching him magic away from the prying eyes of Voldemort. Another could be that they fake their deaths and then work secretly to undermine Voldemort's forces. For example, they could use their knowledge of the Death Eaters to expose their plans and turn some of them against Voldemort.
A really interesting plot could be that James and Lily, alive, work with Dumbledore to create a new school of magic. This school would focus on teaching students how to defend against the Dark Arts in a more practical and hands - on way. Harry would be one of the first students there. They could also develop new magical artifacts that could help in the fight against evil. And the family could have adventures all over the world to find the ingredients and knowledge needed for these new creations.
One interesting plot idea could be that Lily and James, alive and well, decide to train Harry in magic from a very young age. This would make Harry an even more powerful wizard earlier on. They could also use their influence to reform the wizarding world, starting with Hogwarts. For example, they might introduce new courses that focus on both light and dark magic, teaching students how to defend against the dark arts in a more comprehensive way.
In some fanfictions, if Lily and James were alive, they would play a huge role in the fight against Voldemort. They could train Harry and other young wizards in secret. Maybe they'd form an underground resistance group using their knowledge of Hogwarts and the wizarding world to outwit Voldemort and his followers.
One great fanfiction could be 'The Potter's New Life'. In this story, Lily and James Potter are alive and they have to deal with new threats that are different from Voldemort. It shows their parenting skills with Harry in a whole new way, and how they navigate the wizarding world with their new - found life.
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.
Another interesting plot could be that she is transported back in time to the era when her parents were at Hogwarts. She gets to see them as students, experience their school life, and maybe even has a chance to change some small events that could have a big impact on their future and her own present. This would be a great way to explore their relationships and the world of Hogwarts in a different time period.
There are fanfictions where James and Lily alive means that they are part of a time - travel plot. Maybe they are sent back in time to before Voldemort rose to power. They then use their knowledge of the future to prevent the dark wizard from ever becoming a threat. In this kind of fanfiction, they would interact with younger versions of their friends and family, and have to be careful not to disrupt the timeline too much. This creates a really interesting and complex story.
One plot idea could be that Lily's presence changes Harry's upbringing completely. Instead of growing up with the Dursleys, he has a loving home with his mother. Maybe she starts teaching him magic early on, which gives him an edge when he enters Hogwarts. This could also lead to a different dynamic with his friends, as he might be more confident and skilled from the start.
One idea could be that they face challenges from the wizarding world due to their relationship while Lily is pregnant. Maybe there are threats from dark forces who want to harm them or their unborn child.
Another interesting plot could be that they are sent forward in time instead. They have to adapt to the future wizarding world which has changed a great deal. They might be shocked by new magic technologies and different social structures, and their reactions to these changes can make for an engaging story.