Well, a plot in such a fanfiction could involve Harry being betrayed by his friends at Hogwarts. This feeling of betrayal could make him bitter and more susceptible to dark magic. He might then start to seek power from the Dark Arts to get revenge. Also, what if the Ministry of Magic itself turned against Harry in a really unjust way? This could push him to oppose the wizarding world as it is and turn to darker means to fight back.
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.
If Harry turns into a female owl, it would be really cool. Owls are used for mail delivery in the wizarding world. As a female owl, Harry could learn about the inner workings of the owlery and how the owls communicate with each other. He might even be able to eavesdrop on important conversations while on his delivery routes and use that information to help the Order of the Phoenix.
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, 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.
One interesting plot could be Harry Potter, as the Dark Lord, having to balance his newfound power with the relationships in his harem. Each character in the harem might have different motives and loyalties, creating internal power struggles within the group.
Well, a plot could involve a dark magician who has a secret connection to Harry's family. Maybe this dark magician was wronged by the Potters long ago and now wants revenge. This could lead to a complex story where Harry has to uncover the truth about his family's past while also dealing with the dark magician's schemes. The dark magician might also try to manipulate the wizarding world's politics to gain more power and make it easier to target Harry.
One idea could be that a Dark Angel character somehow enters the Harry Potter world and teams up with Voldemort, creating new and dangerous dark magic. Another could be a Hogwarts student discovers they have Dark Angel powers and has to struggle with whether to use them for good or be tempted by the dark side. A third idea is that the Dark Angel realm and the wizarding world are connected through a long - forgotten spell, and characters from both worlds must work together to close the connection before chaos ensues.
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.
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.