One idea could be that Harry discovers he has a sister he never knew about. He then has to protect her from the dark forces in the wizarding world while also teaching her about magic. Maybe they go on a journey to find out more about their family's past together.
Another idea is that Harry, already being a hero, has to teach his sister all about the wizarding world. He shows her Hogwarts, but they face challenges like some students being jealous of her relation to Harry. They go on adventures in the Forbidden Forest together, and Harry imparts his wisdom and courage to her.
One idea could be Teddy Lupin having trouble adjusting to Hogwarts due to his unique situation, and Harry helps him navigate the challenges. Harry could share his own experiences of feeling out of place as a young wizard. For example, Teddy might struggle with his Metamorphmagus abilities and Harry, having faced many odd situations himself, gives him advice on how to control it and not let it make him stand out too much in a negative way.
In this fanfiction, a plot could be Teddy getting into trouble with the Ministry of Magic for some accidental magic gone wrong. Harry has to use his influence and experience to get Teddy out of the situation. Along the way, they could bond over their shared experiences of being different in the wizarding world. Harry could also introduce Teddy to his friends from the old days, like Hermione and Ron, and Teddy could learn valuable life lessons from them. This would not only strengthen their relationship but also help Teddy grow as a person and a wizard.
One plot idea could be that Harry's sister is secretly a powerful witch with unique abilities. For example, she might have the power to communicate with magical creatures in a way no one else can. This could lead to her getting involved in a plot to save a rare and endangered magical creature that is being hunted by dark wizards. Another idea is that she discovers a hidden prophecy related to her and Harry that changes the course of their fight against Voldemort. She could be the key to a new spell or magic that can defeat Voldemort once and for all.
Well, perhaps Harry's sister is a bit of a troublemaker. She could accidentally get into a feud with one of the Slytherin students, and Harry has to step in to keep the peace. Or she could be really good at a subject that Harry struggles with, like Potions, and she helps him improve his skills. She might also have a secret admirer among the students, which could lead to some interesting and humorous situations.
One plot idea could be Hagrid teaching Harry all about the magical creatures in more detail. Since Hagrid has a great love for them, he could take Harry on secret trips to see rare ones. Another could be Hagrid helping Harry deal with the Dursleys in a more magical way, like using minor spells to make their lives a bit more difficult when they're mean to Harry. And perhaps Hagrid could be the one to first introduce Harry to the idea of different magical cultures, as he has so many connections in the wizarding world.
Perhaps Harry's sister was kidnapped as a baby and raised by Death Eaters. She grows up believing in their cause until she discovers the truth about her family. This creates an internal struggle for her. She has to break free from the indoctrination and find a way to reconnect with Harry and the good side. Along the way, she might use the dark magic she was taught in a new and unexpected way to defeat Voldemort, like using a curse against him that he never expected because it was one of his own teachings.
One plot idea could be that the lost sister was separated from Harry during the attack on their parents. She could be raised in a different magical family, and when she discovers her true identity, she has to navigate between her new family and her connection to Harry. Another could be that she was hidden away by Dumbledore for her own protection and has unique magical abilities that Voldemort wants to use. When she enters the wizarding world, she becomes a target but also a powerful ally to Harry.
One interesting plot idea could be that Draco's sister is secretly a supporter of the Order of the Phoenix. She might be in a difficult position, having to hide her true beliefs from her family while trying to help the good side in the wizarding war. Another plot could involve her having a special magical ability that she has to keep hidden from everyone, including Draco. Maybe she can communicate with magical creatures in a way no one else can, and this ability ends up being crucial in a battle or a quest to save Hogwarts.
One idea could be that the twin sister has a special power that Harry doesn't. For example, she can communicate with animals. This would add a new dynamic to the story as she could use this ability to gather information or help in dangerous situations. Another plot could be that the twin sister gets sorted into a different house than Harry at Hogwarts. This would cause some tension and also show different perspectives within the wizarding world. A third idea is that the twin sister discovers a hidden secret about their family that Harry is oblivious to, and she has to figure out how to tell him and what to do about it.