Maybe a spell goes wrong and creates a baby that has a combination of their magical traits. They then need to figure out how to reverse the spell while also dealing with the sudden 'parenthood'. They might discover new sides of themselves as they care for this unique baby.
One idea could be that Harry and Draco find an abandoned magical baby and they have to work together to take care of it. This forces them to put aside their differences and learn about each other in a new way.
One idea could be that Harry and Draco find a magical toy that has the power to transport them to different wizarding worlds. They have to work together to find their way back home. Another could be that they both inherit a collection of enchanted toys from an old wizard, and they must figure out how to use the toys to solve a mystery in Hogwarts.
One idea could be a story where Harry, Draco, and Severus are forced to work together on a dangerous mission to save Hogwarts from a new dark threat. They start off distrusting each other but gradually learn to respect and rely on one another.
One idea could be a story where Draco, Harry and Hermione are forced to work together on a special magical project for the Ministry. They start off hating each other but gradually come to respect one another's skills. Another plot could involve them getting trapped in an alternate magical dimension where they have to rely on each other to find their way back. Maybe Draco has some knowledge of the dark magic that created the dimension, Harry has his Gryffindor courage to lead them through dangerous areas, and Hermione uses her vast knowledge to figure out how to break the spell that trapped them.
Maybe the marriage law causes a stir in the wizarding world. Draco and Hermione initially resist but then realize that for the sake of the baby, they need to make it work. The baby might be born with a special gift that is a combination of the Malfoy and Granger magic. They could have to deal with Draco's family not accepting Hermione at first, but the baby becomes the bridge that brings them all together in the end.
One idea could be that as a baby, Harry has some adult - like wisdom and he starts to subtly influence the adults around him in Hogwarts. For example, he might give Dumbledore some deep insights about the future of the wizarding world that Dumbledore hadn't considered before.
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 Draco and Harry are forced to work together secretly in the Chamber of Secrets to solve a mystery that Dumbledore suspects is bigger than just the basilisk. They find a hidden chamber within the Chamber of Secrets that contains a powerful magical artifact from the founders' time, and they must figure out how to use it to save Hogwarts.
A possible plot is a mystery - solving one. There's a strange magical occurrence in Hogwarts that is affecting both Gryffindors and Slytherins. Hermione starts investigating, and she enlists the help of Sirius (who could be a ghost or somehow back in a semi - corporeal form). Harry is naturally drawn to help Hermione, and Draco gets involved because it might affect his family's standing. As they dig deeper, they find out that it's an ancient curse set by a long - forgotten wizard, and they have to find a way to break it using their combined skills and knowledge.
Well, perhaps they meet in a hidden part of Hogwarts, like an old abandoned classroom. They start exchanging secret notes and small gifts, but they're constantly worried about getting caught. There could be a plot where Draco's family starts to suspect something and he has to work hard to keep Harry a secret from them. It would add a lot of tension and drama to the story.
A possible plot for this fanfiction could be that Voldemort is still a threat, and they hide their relationship to protect each other. If Voldemort or his Death Eaters were to find out about their relationship, they could use it against them. So they keep it a secret even from some of their closest allies. They might also have to deal with the internal struggle of wanting to be open but knowing the risks are too great. This could lead to some very emotional and tense scenes in the fanfiction.