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 interesting plotline could be a wizard lawyer defending a wrongly accused magical creature. For example, a hippogriff that was attacked by a wizard but is being blamed for the incident. The lawyer has to gather evidence in the wizarding world, which might involve spells like Revelio to uncover hidden facts and interviewing witnesses like Hagrid who has a deep knowledge of magical creatures.
One possible plotline could be a private lawyer helping Harry Potter deal with the legal aspects of his inheritance from his parents. Maybe there are some dark wizards trying to contest it, and the lawyer has to use both magic and Muggle - like legal knowledge to fight for Harry's rights.
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 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.
A possible plotline could be that Harry uses his invisibility to spy on Voldemort's followers and gather intelligence for the Order of the Phoenix. This would add an element of mystery and danger as he has to be extremely careful not to be detected.
A possible plotline could be that lithium is a key ingredient in a new spell that a young wizard discovers. This spell could be used to combat the dark forces in a unique way. Another plotline might be that lithium is found in a hidden part of the Forbidden Forest and it has some sort of connection to an ancient magic that was long forgotten.