In some fanfics, the sister could be a key to unlocking an ancient magic that has been forgotten. Her presence in Hogwarts might stir up old secrets and prophecies. The story could revolve around how she and Harry work together to figure out this new magic and use it to defeat the dark forces.
One interesting plotline could be how the twin sister helps Harry deal with the challenges at Hogwarts in a different way than his friends. Maybe she has a unique magical ability that aids them in fighting the dark forces.
A more complex plotline could involve the sister being targeted by Voldemort because he sees her as a threat to his plans. She might have a prophecy associated with her that predicts she will be the one to finally defeat him. This would put her in a lot of danger and force Harry to be even more protective of her while also relying on her unique skills to fight against the Dark Lord.
In some fanfictions, Harry's sister might be sorted into a different house than him, like Slytherin. This would create a lot of internal conflict within the family as well as at Hogwarts. It could lead to a situation where Harry has to defend his sister against the prejudices of his friends. Or perhaps she has a prophecy related to her that ties into Harry's own destiny, and they have to figure out how to fulfill both prophecies without sacrificing one another.
One interesting plotline could be the twin sister having a unique magical ability that Harry doesn't. For example, she might be able to communicate with magical creatures in a way no one else can. This could lead to her getting involved in adventures that are separate from Harry's at Hogwarts, but still end up being connected in some way, like when she discovers a secret about a creature that is related to a dark wizard who is also after Harry.
Well, one plotline could be the main character, perhaps a new healer at Hogwarts, dealing with magical maladies that are beyond the usual. For example, they might encounter a spell - induced illness that spreads among the students and has to find a unique cure using both traditional and forgotten healing methods.
Perhaps in the fanfic, Harry is turned into a snake for a period of time. As a snake, he experiences the wizarding world from a different perspective. He has to learn how to survive as a snake while still trying to fulfill his role in the fight against Voldemort. He might encounter other snakes that are either friendly or hostile, and he has to use his wits to deal with them.
Since this is an improper concept for the Harry Potter world, there are no so - called 'good' plotlines in such fanfic. We should uphold the positive values that J.K. Rowling intended in the original work, rather than exploring such unseemly fan - created ideas.
Well, perhaps in 'the king harry potter fanfic', there's a plotline where Harry discovers a long - lost magical artifact that gives him the power and status of a king. This could lead to him having to face off against old enemies who now see him as an even greater threat. And his friends from Hogwarts might have to band together to protect him as he adjusts to this new role. Also, it could involve new magical creatures being drawn to him because of his new - found king - like aura.
A great plotline in some fanfics is the discovery of new spells and magic. It could be that a long - lost spellbook is found, and the characters start to experiment with spells that are more powerful and dangerous than what they are used to. This could lead to all sorts of situations, from accidentally creating new magical disasters to using these spells to fight off new, unexpected enemies.
One interesting plotline could be Voldemort being redeemed. It's a really different take as in the original books he's pure evil. But in fanfic, some writers explore what if he had a change of heart.