One interesting plot line could be how the twin's presence affects Harry's relationships at Hogwarts. Maybe the twin has a different personality and befriends characters that Harry doesn't usually interact with much, like some of the Slytherins. This could lead to a whole new set of adventures and alliances within the school.
Another possible plot line is that the twin gets sorted into a different house than Harry. This would create a divide between them in some ways, but also an opportunity for them to learn about each other's houses and break down some of the traditional house rivalries. They could then work together to bring the different houses closer, using their combined knowledge and skills from their respective houses.
Maybe the twin sister is sorted into a different house than Harry. This would cause a bit of a rift at first as they have to adjust to different house cultures. But in the end, they learn to combine the strengths of their respective houses to overcome a great threat to Hogwarts.
One interesting plot twist could be that the forgotten twin has a different magical ability than Harry. For example, while Harry is good at Defense Against the Dark Arts, the twin might be a natural at Potions or Charms. This could lead to a situation where the twin is initially seen as an outcast in Hogwarts because their skills don't match the typical Gryffindor traits. Another plot twist could be that the forgotten twin was actually hidden away by a powerful dark wizard who planned to use the twin's power for their own evil purposes. This would put the twin in a difficult position when they discover their true past and have to decide whether to side with the forces of good or be influenced by the dark wizard's plans.
A possible plotline is that the twin was separated from Harry at birth for some mysterious reason. Maybe a dark wizard wanted to use the twin's power for his own evil purposes. Years later, when they meet at Hogwarts, they don't recognize each other at first. But as they start to get to know each other, they sense a strange connection. This discovery could then lead to a journey of uncovering their past and their true family history. Along the way, they would face many challenges together, both from within the school and from the outside forces that are still interested in the twin's power.
One interesting plot line could be that the Potter twins have to go on a quest to find a lost magical artifact that has the power to change the fate of the wizarding world. Along the way, they make new friends and face many challenges. Another could be that they get involved in a time - travel adventure related to their parents' time at Hogwarts and end up changing some things in the past which then has consequences in the present.
One interesting plotline could be the younger twin having a unique magical ability that even Harry Potter doesn't possess. For example, the ability to communicate with magical creatures on a deeper level.
In some 'Harry Potter is a pilot' fanfictions, there could be a plot where Harry has to transport magical creatures by air. He'd have to figure out how to keep them calm and safe during the flight. Also, perhaps he teams up with other wizard pilots to form a sort of magical air force to protect the wizarding world from threats in the air.
She could be the key to unlocking some long - forgotten secrets about their parents. Maybe she discovers a hidden diary or a magic object that reveals things about their family's past that even Dumbledore doesn't know. This could put her in danger as there might be dark forces who don't want those secrets to come out. Harry then has to protect her while they figure out what to do with the new - found information.
A cool plot could be that the twins cross over into a dimension where they are sorted into different houses. Fred in Slytherin and George in Gryffindor. This would create an interesting dynamic as they deal with the different values and expectations of their new houses while still maintaining their close bond. They could also have to face the prejudice and misunderstandings from their housemates due to their 'unusual' sorting.
Well, obviously Harry Potter and his twin are the main characters. Harry is known for his courage and his connection to Voldemort. His twin, depending on how the fanfic is written, could be someone with a different set of traits, like being more studious or more outgoing. Then there would also be characters like Ron and Hermione who are likely to be involved in the story. They are Harry's best friends in the original series and would surely have a role to play in the fanfic as well, whether it's helping the twins with their adventures or dealing with the complications that the twin's presence brings.
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.