A really interesting plot for a fanfic where Hermione is Harry's sister could involve the two of them having a shared magical inheritance from their parents that they need to figure out. This could lead to adventures where they explore old family magic. It could also change the dynamic of their relationships with Ron and other friends. Hermione might be more protective of Harry in this scenario, and they could have inside jokes and a deeper understanding of each other based on their family bond.
A really interesting plot could be that the reborn character is actually a huge Harry Potter fan in their previous life, so they know all the secrets and prophecies. They use this knowledge to their advantage. For instance, they might be able to figure out how to defeat Voldemort more quickly by using a combination of things they remember from the books and their own new ideas. They could also change the relationships between characters. Instead of Harry and Hermione just being friends, the reborn character could nudge them towards a romantic relationship earlier on, which would then have a ripple effect on the whole story.
They could start a magical school of their own. As a married couple, they would pool their resources and knowledge. Hermione's love for learning and Harry's practical experience would make for a great curriculum. They would face opposition from the traditional wizarding schools at first, but eventually prove that their new way of teaching magic is beneficial for young wizards and witches.
One idea could be that Harry and his sister discover a hidden magic within their family heritage that no one else knew about. They could go on an adventure to explore this new magic and its implications.
One idea could be an OC who is a slave in the wizarding world but discovers they have a unique magical ability that could change their status. For example, they might be able to communicate with magical creatures in a way no one else can, and this ability catches the attention of Harry Potter and his friends. They then help the OC fight against the unjust system of slavery in the wizarding world.
One possible plot idea could be the sister getting involved in a charity project that unites the royal family and the public in a new way. Maybe she takes on a role in promoting environmental causes, like starting a campaign to clean up the local parks near the royal estates.
A plot could be that the Triwizard Tournament tasks are magically altered to target Harry specifically, and Hermione has to use her skills to figure out who is behind it. Maybe a new dark magic is involved that she has to research. She then has to train Harry in new ways to face the challenges. For example, teaching him some ancient spells that are not commonly known.
One idea could be that during a dangerous adventure, Harry realizes his true feelings for Hermione when she risks her life to save him. He then starts to court her in a very Harry - like way, with shy glances and small, kind gestures.
A possible plot is that a spell gone wrong makes Queen's music the only thing that can calm a dangerous magical creature. Hermione, being the clever witch she is, uses her love for Queen to figure out how to tame the creature and in the process, she also uncovers a long - lost magical connection between Queen's music and ancient wizarding magic.
Well, perhaps in the fanfic, Harry and Hermione find out that the dragons are actually magical guardians protecting a powerful ancient artifact within the tournament arena. They then have to decide whether to compete against the dragons as the rules state or try to work with them to uncover the mystery of the artifact. And also, the dragon that Harry is supposed to face in the tournament could have a special connection to his past that Hermione helps him uncover.
One idea could be a story where baby Harry and Hermione are magically transported to a different time or place. They could be in a magical forest full of strange creatures that they have to befriend in order to find their way back. Another plot could involve them as babies having a special connection that allows them to communicate without speaking. This could lead to them solving small mysteries in Hogwarts even at a very young age. A third idea might be that they are found by a group of magical beings who don't know who they are and raise them with different magical traditions, and later they discover their true identities.