One plot idea could be that they are secretly helping each other in their respective tasks. For example, Draco might be secretly passing information to Harry about some Death Eater plans while pretending to be on their side still. Another could be that they discover a hidden magical bond between them that draws them together in secret. They might meet in a secluded part of Hogwarts, like the Room of Requirement, to explore this bond without others knowing.
A possible plot is that they are forced into a secret relationship due to a magical curse. The curse makes it so that if their relationship is discovered, something terrible will happen to Hogwarts or their loved ones. They start off resenting each other because of their past differences but as they spend more time together in secret, they start to fall in love. They have to be extremely careful, even using spells to cover their tracks when they are together. For instance, using a disillusionment charm when they meet in the Forbidden Forest.
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.
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 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 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.
Another plotline could involve the challenges they face due to their different backgrounds. Harry being a Gryffindor and Draco a Slytherin, their houses have long - standing rivalries. They would need to overcome these differences and the prejudices of their fellow students.
Well, perhaps Draco, in the role of Merlin, has a special connection to the magical creatures that he uses to help Harry. Harry, as Arthur, is trying to unite the different factions of the wizarding world. Draco's knowledge of magic, similar to Merlin's, allows him to create magical artifacts or spells that assist Harry in his quest for unity. And there could be a love story involved too, like Guinevere and Arthur, but in the context of the wizarding world with some other character and Harry.
Another idea is that the OC is a Muggle - born with an amazing talent for potions. They end up in a class project with Harry, Draco, and Blaise. Their different approaches to the project (Harry's courage, Draco's ambition, Blaise's shrewdness) and the OC's out - of - the - box thinking create a really interesting dynamic as they compete and also learn from each other.
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.