Well, a storyline could revolve around the mystery of her blindness. Maybe it was caused by a dark spell when she was a baby, and as the story progresses, Harry and his friends try to find a way to reverse it. Meanwhile, she is learning to be independent and strong. She could also form a special bond with some magical creatures that are attracted to her unique situation. For instance, a thestral might befriend her because she can sense its presence without seeing it in the normal way.
A great storyline could be her being a key to a hidden prophecy. Since she is often overlooked because of her blindness, the bad guys don't realize her importance at first. But it turns out that her blindness gives her a different perspective on the prophecies. She might start having strange dreams related to the future of the wizarding world. Harry then has to protect her while also trying to figure out what her dreams mean and how they can use this knowledge to defeat the dark forces.
One possible storyline could be a blind Harry Potter being magically transported to the Avengers' universe. There, he might use his magical skills in unexpected ways to help the Avengers. For example, his knowledge of defensive spells could be used to protect the team from new and strange threats that their technology can't handle.
One interesting storyline could be that Harry discovers his moon powers during a full moon at Hogwarts. Maybe he can communicate with magical creatures that are only active during the full moon. Another could be that his moon powers are linked to a hidden prophecy, and Voldemort wants to steal those powers for himself.
One interesting storyline could be that his younger siblings are sorted into different houses at Hogwarts. For example, one might be in Hufflepuff while Harry is in Gryffindor. This could lead to some sibling rivalry as well as moments of them coming together to face challenges. Another could be that his younger siblings have unique magical abilities that Harry doesn't possess, and they end up helping him in unexpected ways during his battles against Voldemort.
Perhaps a fanfiction could show Harry's relationship with his professors in a new light. Some professors, like Professor Snape, might initially be unsympathetic, seeing it as a Muggle weakness. But as the story progresses, they could come to understand the gravity of his situation and help him in their own ways. Dumbledore, on the other hand, could use his wisdom to guide Harry in accepting his condition and still being a great wizard.
One possible storyline could be that Harry discovers his angel wings during a dangerous battle at Hogwarts. The wings give him the ability to fly faster and more gracefully than a broomstick, allowing him to outmaneuver the dark forces. He could use them to rescue his friends from precarious situations, like flying them to safety when the castle is under siege.
Another storyline could involve the sister being in a different magical school all this time. When she discovers Harry's situation, she transfers to Hogwarts. She then tries to fit into the Hogwarts environment while also trying to figure out how to use her knowledge from her previous school to aid Harry. She might bring new ideas about magic that are different from what they teach at Hogwarts.
She might have a love story with a Gryffindor boy. This would be a huge scandal in the wizarding world considering the long - standing feud between the two houses. Their relationship could start as a secret, with them meeting in hidden places in Hogwarts like the Room of Requirement. As they fall deeper in love, they face many challenges, not just from their families and houses, but also from the dark forces at play in the wizarding world. Her love for this Gryffindor could be the catalyst for her to question and ultimately break away from the dark ways of her family.
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.
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 possible plotline could be that the long lost sister was separated from Harry due to a powerful dark wizard's curse. She might have been raised in a different magical community, like a far - flung wizarding village in Scotland. When she discovers her true identity, she has to quickly adapt to the wizarding world Harry is familiar with, facing challenges like learning about Hogwarts and its traditions.