In a fanfiction, it could be that a magical accident binds Harry and Narcissa's magic together. At first, they are both horrified, but as they try to figure out a way to break the bond, they start to understand each other better. Harry realizes that Narcissa was just a mother trying to protect her son in a difficult situation. Narcissa sees Harry's courage and selflessness. Their relationship evolves from forced proximity to true love, and they decide to get married, defying all the norms of the wizarding society.
One possible plot could be that in a world where the war ended differently. Harry, having seen Narcissa's true nature in saving him at the end, starts to interact with her more. They find common ground in their love for Draco and a desire to heal the wizarding world. Their relationship slowly blossoms from there, with Harry helping Narcissa come to terms with a new world order, and Narcissa teaching Harry more about the pure - blood traditions in a non - prejudicial way.