Maybe Daphne and Harry have a shared secret about a family heirloom. As they grow up, they realize this heirloom has a much deeper significance in the battle against Voldemort. Their childhood friendship gives them the trust to explore its powers together.
In the fanfiction, it could be that they were both outcasts in their own ways as children. Harry because of his Muggle - raised status and Daphne perhaps for not conforming to the pure - blood Slytherin norms. Their friendship was born out of this mutual understanding, and as they get older, they use their combined skills to change the views in Hogwarts.
One plot idea could be that they discover a hidden magical place in their childhood and later use it to hide from the Dark forces in the wizarding world. They might have made a pact as children to protect this place.
One plot idea could be that Harry and Daphne are both orphans in the wizarding world, though Harry's situation is more well - known. They meet at an orphanage for magical children. Daphne is often overlooked because she is a girl and from a family that has some dark rumors. Together, they discover a hidden room in the orphanage that has a magical portal to a world full of strange and wonderful magical creatures. This shared discovery bonds them and they start to rely on each other for support and friendship.
One idea could be that they initially resist the betrothal. Harry is suspicious of Daphne because of her Slytherin heritage, and Daphne thinks Harry is just a fame - seeking Gryffindor. But as they are forced to spend time together for various magical tasks assigned by their families, they start to see each other's true natures and gradually fall in love.
Maybe there could be a plot where they face opposition from some quarters. Some might not approve of a union between Harry and Daphne due to their houses' historical rivalries. They would then have to overcome these prejudices and prove that their love is stronger than the old grudges. This could involve them going on a quest together to show that they are a united front and are meant to be together.
One idea could be that they meet in a magical library. Harry is researching a rare spell and Daphne is there looking for knowledge on potions. Their eyes meet, and from that moment, a connection begins. They start secretly meeting to exchange their findings, and slowly, their relationship blossoms into love, leading to marriage.
One idea could be that Harry and Daphne's baby has some unique magical abilities that are a combination of their genetics. For example, the baby might be able to communicate with magical creatures in a way that neither Harry nor Daphne can. Another plot could be centered around the challenges they face as new parents in the wizarding world, like dealing with over - protective relatives or the paparazzi - like wizarding press.
One idea could be that they discover a hidden magical place in the grounds of Hogwarts as children. They keep it a secret from everyone else and it becomes their special retreat. They meet there to share their thoughts, Hermione's love for books and Draco's family pressures. It could also be a place where they practice magic away from the prying eyes of teachers and other students.
One idea could be that Harry and Daphne are paired up for a special project at Hogwarts. As they work together, they start to see each other in a new light. Daphne might initially be a bit cold towards Harry, but as she gets to know him better, she warms up. Their relationship could develop slowly as they face challenges within the school, like dealing with a new dark magic threat.
One interesting plot could be Daphne discovering a hidden secret passage in the Chamber of Secrets that even Harry doesn't know about. She could then team up with Harry to explore it further.
One idea could be the challenges they face in accepting the betrothal at first. Harry might be hesitant due to his loyalty to his friends and the wizarding world's expectations. Daphne, on the other hand, could be dealing with her family's pressure and her own feelings towards Harry.
Imagine if the Dementors from Harry Potter somehow end up in New York where Friends is set. The Friends gang has to deal with the effects of the Dementors, like feeling all their happiness being sucked out. Phoebe, with her positive and somewhat otherworldly nature, could be the one to figure out how to repel them, maybe through her music or some kind of new spell she discovers.