One possible plot could be that Draco discovers a prophecy related to Harry's magical cancer. This prophecy might suggest that Draco has a role to play in either saving or dooming Harry. He has to then grapple with his own destiny and his complex relationship with Harry. Along the way, he might also face opposition from his own family and other dark wizards who don't want Harry to be saved.
In such a fanfiction, Draco might initially be in denial about Harry's condition. But as time passes, he realizes the gravity of the situation. He could then embark on a journey to understand the nature of this magical cancer, which might lead him to some dark and forgotten corners of the wizarding world. This could also be a story of redemption for Draco, as he tries to save the life of someone he once considered an enemy.
The theme of mortality would be prominent. Harry's impending death due to magical cancer makes everyone, especially Draco, confront the fragility of life in the wizarding world. Also, the theme of self - discovery for Draco as he delves into this mission to save Harry and discovers new aspects of himself and his capabilities.
One possible plotline could be Harry gradually losing his magic as the 'magical cancer' progresses. He might struggle to keep it a secret from his friends, especially Hermione and Ron, who would be desperate to find a cure. Another could be that he starts having strange visions or experiences related to the magical source of this 'cancer' which could lead him on a quest to find its origin in the wizarding world, even as his health deteriorates.
They would all come together to create a special environment for Harry. They might set up a room in Hogwarts filled with all the things he loves, like pictures of his parents, his Firebolt, and his invisibility cloak. They would also try to organize magical events, like mini - Quidditch games in the Hogwarts grounds just for him, to keep his mind off his illness as much as possible.
Draco may also use his connections in the wizarding world to find the best healers for Harry. He could pull some strings to get Harry into experimental treatments or get second opinions from the most renowned magical doctors. In this way, Draco uses his influence and resources to give Harry the best chance at fighting the cancer.
If Harry is Salazar, he might have enhanced Parseltongue abilities. Salazar was known for his connection with snakes, so Harry could communicate with a wider variety of serpents and perhaps even gain more control over them. For Draco as Godric, he could have a stronger affinity for Gryffindor - like magic, such as a more powerful Patronus, perhaps in the form of a lion, representing Godric's courage.
Some fanfictions in this vein might explore the idea of legacy. Harry, knowing he's dying, could be concerned about how he'll be remembered. He might try to pass on his knowledge and values to the younger generation at Hogwarts. It could also show the impact of his illness on his relationships. His relationship with Ginny might be strained as they both struggle to deal with the reality of his situation.
Fanfiction writers make it believable by showing the reactions of the other characters. If Hermione is researching frantically, and the doctors in the wizarding world are stumped, it gives the impression that this is a serious and real situation. Also, by making Harry's character arc focus on his acceptance of his fate, it mirrors what many real - life cancer patients go through, which helps the reader believe in the story.
One great fanfiction in this area is 'The Creature Within'. It delves into how Harry and Draco have to work together to save a rare magical creature from being exploited. The story beautifully weaves in their complex relationship with the magical creature's plight.
Perhaps Draco could be a Kneazle. Kneazles are intelligent and independent, which would suit Draco's personality. As a Kneazle, he might be constantly sneaking around and getting into places he's not supposed to. Harry would have to use his wits to keep Draco out of trouble and also earn his trust, as Kneazles can be quite picky about who they bond with.
Well, Draco could be very conflicted. On one hand, he has his own pride and his Slytherin upbringing which might tell him to stay away. But if he has developed deeper feelings for Hermione in the fanfic, he might put aside their past differences. He could start by using his family's resources to find a cure or at least better treatments for her cancer. He may also become more protective of her, keeping her away from those who might be unkind about her illness.