There's also the theme of sacrifice. Sometimes Erik sacrifices his own desires for Christine's happiness, and vice versa. It adds a layer of depth to their relationship. And the exploration of their different social standings, with Erik being an outcast and Christine being a more 'normal' part of society, is another common theme in these fanfictions.
Love and passion are very common themes. Since lemon often implies a certain level of romantic or sexual tension, these fanfictions usually explore the deep love between Erik and Christine in a more intense way.
Love and redemption are common themes. Since Erik has a troubled past, in many fanfictions, Christine's love redeems him. For example, in some stories, he learns to be more open and less vengeful because of her love.
One common theme is redemption. Erik often seeks redemption for his past actions through his relationship with Christine. For example, in many stories, he tries to prove that he can be a better person because of his love for her.
Love is a very common theme. It could be the unrequited love that Erik has for Christine at first, or the evolving love between them. Another theme is redemption. Fans often write about Erik's journey towards redemption through his relationship with Christine. Also, the idea of acceptance, like Christine accepting Erik for who he is, is quite common.
Redemption is also a theme. Erik, being a complex and often misunderstood character, may seek redemption through his relationship with Christine. Fanfictions might explore how he tries to make up for his past wrongdoings, such as his threats and manipulations in the opera house.
One common theme is redemption. Often, Erik is depicted as seeking redemption through his love for Christine. Another theme is the power of music, as it is a strong connection between them. For example, in many fanfics, Christine's singing can soothe Erik's tortured soul.
In a different version, Christine gets lost in the opera house and Erik is the one who finds her. He could have been watching her from afar all this time. Instead of scaring her, he gently leads her to a safe place. They start talking about their dreams and fears. Christine realizes that Erik is not the monster everyone makes him out to be. This new understanding blossoms into love as they continue to meet secretly and share their innermost thoughts.
A simple fanfiction could be that Erik writes a beautiful song for Christine. When she hears it, she is deeply moved. She seeks him out to thank him. This encounter starts a series of meetings where they slowly fall in love. Their love is based on mutual respect for each other's talents.
Christine might initially be very frightened. After all, being kidnapped is a terrifying experience. She could scream and try to fight back.