One common plot device is the redemption arc. Narcissa may be portrayed as turning away from the dark side, and Harry, seeing this change, is drawn to her. Another is the protection plot, like Narcissa protecting Harry for some reason, which then leads to a bond between them. And sometimes, there's a magical bond or curse that forces them to interact and eventually fall in love.
Well, a frequent plot device is the secret alliance. Harry and Narcissa might form a secret pact to achieve a common goal, like saving Draco or undermining Voldemort's plans. As they work together secretly, they start to develop feelings for each other. Also, the memory - sharing device can be used. Maybe through some magic, they share their deepest memories and fears, and this new - found understanding makes them fall in love. There's also the idea of a prophecy or a fated encounter that sets the stage for their relationship to develop.
Common plot devices include the power of forgiveness. Harry, being the forgiving character he is, might forgive Narcissa for her past actions, and this act of forgiveness could lead to love. Another is the idea of a shared destiny. In fanfiction, they could be fated to be together to bring about a new era in the wizarding world. Additionally, the trope of the outcast - the idea that both Harry and Narcissa feel like outcasts in their own ways within the wizarding society can be used. They find solace in each other, and this mutual understanding leads to love.