Maybe Harry discovers some dark family secrets that make him question his place on the 'good' side. He could find out that his ancestors had strong ties to dark magic, and this knowledge could slowly corrupt him. He might start to experiment with dark spells, thinking that he can control them and use them for what he believes is the greater good, but gradually slips towards the darkside.
In a fanfiction like this, Harry could be influenced by a powerful dark wizard who manipulates his emotions. This wizard could prey on Harry's feelings of loneliness and the loss of his parents. By making false promises of power and a way to bring his parents back, he lures Harry towards the darkside. Harry, in his desperation, starts to believe in this wizard's lies and begins to embrace the dark magic.
One possible plotline could be that Harry discovers a dark secret about his family and feels betrayed by the wizarding world. This could lead him to side with the dark forces. Another could be that he is influenced by a powerful dark wizard who manipulates his mind. Maybe he becomes envious of the power others hold and decides to take it by any means, turning evil in the process.
It could still be Voldemort. Even if Harry is turning to the darkside, Voldemort might see him as a threat because Harry has his own unique power. Voldemort might not want to share his control over the dark forces with Harry, so he would become the main antagonist in a fight for dominance in the dark world.
One possible plotline could be that Lily, now as a girl, has to navigate the wizarding world all over again. She might face different challenges and opportunities than she did before. For example, she could form new friendships or rivalries at Hogwarts.
One plotline could be Naruto being a transfer student at Hogwarts. He might struggle to adapt to the magic system while also showing off his ninja skills. Another could be Naruto teaming up with Harry Potter to fight a common enemy that threatens both the ninja world and the wizarding world.
One possible plotline could be Harry using his knowledge of the wizarding laws, which he learned through his years at Hogwarts and his experiences with the Ministry of Magic, to defend wrongly accused wizards in a wizarding court. Another could be him trying to reform the wizarding legal system which he might find full of loopholes and biases.
One possible plotline could be Harry trying to hide his merman identity while at Hogwarts. He might face challenges in the magical underwater areas of the school, like the lake. Another could be that he discovers his merman heritage and has to go on a journey to learn more about it and his new powers.
One possible plotline could be Harry being secretly trained by an unknown figure to become an assassin against the dark forces. He would have to hide this new identity from his friends at Hogwarts.
One possible plotline could be about the Weasley family trying to improve their financial situation. Maybe they come up with a new business idea in the wizarding world, like a magical product that becomes a huge success and changes their economic status. Another could be a story about how the Ministry of Magic regulates the financial system in the wizarding world, and there are some characters who try to manipulate or reform it.
In some fanfictions, the Queen might be a long - lost relative of a pure - blood wizarding family. This would lead to her being involved in the complex politics of the wizarding world. She could use her position as the Queen to try and bring about change in the wizarding world's old - fashioned pure - blood - supremacy - based society.
One possible plotline could be a Lannister student at Hogwarts. They might bring their family's ambition and power - hungry nature into the school, causing all sorts of drama among the houses. For example, trying to take over the Slytherin house leadership through manipulation.