One interesting aspect could be the origin of such a dark dragon. It could be a result of dark magic experiments gone wrong. For example, a wizard trying to enhance a dragon's natural abilities with dark magic, but instead creating a more menacing and uncontrollable creature. This would then lead to a whole new set of challenges for Harry and his friends to face.
Obviously, Harry Potter himself is often involved. He's the hero of the original series, so in fanfiction about a dark dragon, he'd be the one to step up and face the new threat. Hermione and Ron would likely be by his side too, as they are his closest friends.
One idea could be exploring the backstory of a Dementor. Maybe it was once a wizard who dabbled in dark magic too much and was cursed to become that form. Another could be a story about a werewolf who tries to find a cure in the wizarding world outside of the Wolfsbane Potion. And also, a fanfiction about a Basilisk that has a secret connection to an ancient wizarding family.
Another interesting fanfiction idea would be to have a character from The Dark Crystal, like Jen, end up at Hogwarts. Jen could learn about magic and how it differs from the magic in his own world. He might form friendships with the Hogwarts students and together they could face a common threat that somehow links the two universes, like a dark force that wants to take over both the wizarding world and the world of The Dark Crystal.
One possible plot idea could be the Dark Prince having a hidden agenda that not even Voldemort is fully aware of. He might be secretly manipulating events to gain ultimate power for himself. For example, he could be using the chaos of the wizarding war to his advantage by making deals with both sides without their knowledge.
One idea could be exploring how the Dark Mark affects the mental state of those who bear it. For example, it might cause nightmares or hallucinations related to Voldemort's will. Another could be a story about a character trying to hide their Dark Mark from their friends in the Order of Phoenix, creating a lot of internal conflict and secrecy.
One idea could be that Harry discovers a dark secret about his family that leads him down a dark path. For example, he might find out that his ancestors were involved in some really evil magic and he starts to explore that side of magic. Another could be that the influence of Voldemort somehow remains and corrupts Harry's mind over time. Maybe the Horcrux inside him wasn't completely removed and it starts to change his way of thinking.
One idea could be that Harry, influenced by dark magic, forms an uneasy alliance with Voldemort. They could team up to take over the wizarding world in a different way than Voldemort originally planned, perhaps by ruling jointly. Another plot could involve Harry exploring the darker aspects of his own magic and using some of Voldemort's knowledge to do so, which causes a rift between him and his friends.
One idea could be Harry, as a dark necromancer, using his newfound powers to bring back some of the great wizards from the past to learn from them, but the magic has unforeseen consequences like attracting dark entities. Another could be that he's on a quest to revive a loved one, which goes against the natural order of magic in the wizarding world and causes a rift between him and his friends.
One interesting plot idea could be a story where a student discovers a long - lost dark arts spell book in the Forbidden Forest and has to resist the temptation to use the powerful and dangerous spells within it. This could lead to a battle between good and the allure of the dark arts within themselves.
In a dark fanfiction, Voldemort could have placed a dark spell on Harry long before he knew. As Harry grows, the spell starts to take effect, making him darker. Luna, being an outsider in many ways, notices the changes in Harry. She decides to research ancient magic to find a way to break the spell. Along the way, she discovers secrets about her own family's magic that could be the key to saving Harry and the wizarding world from the impending darkness.