It would be a very strange and interesting story. Since Lily is Harry's mother in the original story, it would have to involve some kind of time - travel or alternate - reality plot. Maybe Harry goes back in time and changes things so that he can be with Lily. It would be full of drama and emotion as they deal with the implications of their relationship.
Well, in such a fanfiction, it would likely start with a major divergence from the original plot. For instance, there could be a magical accident that sends Harry back to a time when Lily was alive. Their relationship would be complex. Harry would have to reconcile his knowledge of her as his mother with his new - found romantic feelings. And the story would also need to deal with how this new relationship affects the wizarding world around them. There would be a lot of conflict, not only from the characters themselves but also from the people around them who might find their relationship highly unusual.
A 'Harry Potter married Lily Evans' fanfiction would be a wild ride. It would require a complete re - imagining of the relationships and the timeline of the Harry Potter universe. It might start with a magical anomaly that disrupts the normal flow of time. Harry, being in a different time period, would be attracted to Lily for her courage, intelligence, and kindness. But they would face numerous challenges. How would they explain their relationship to others? How would it change the events that were supposed to happen? It would be a story full of love, confusion, and a struggle to fit into a world that has been turned upside down by their relationship.