You can also explore the impact on the relationships within the Vampire Diaries universe. Maybe the sister was in a relationship before the amnesia. Now, her partner has to deal with her not remembering their love. It could lead to some really interesting plot twists, like the partner having to fall in love with her all over again while she discovers new aspects of herself.
A plot idea could be that the sister has amnesia due to a vampire - related incident. Maybe she was bitten by a new type of vampire that causes memory loss. The other characters then have to find a cure while also protecting her from other threats in the vampire world.
One common theme is loyalty. Sisters in the fanfiction often show unwavering loyalty to each other despite the supernatural challenges they face. Another is sacrifice. They might be willing to sacrifice their own happiness or safety for their sister. For example, in many stories, a sister will put herself in danger to protect her sibling from a vampire or a witch's curse.
In Vampire Diaries fanfiction, the concept of a twin sister sacrifice can add a whole new layer of drama. It could be related to some dark magic ritual where a twin's sacrifice is needed to gain power or break a curse. For example, perhaps a powerful vampire or witch demands it to achieve immortality or to control the supernatural elements in Mystic Falls.
One common theme is family loyalty. The sisters often have to stick together in the face of the many threats in the vampire - filled world. Another is power struggles. The sisters may have unique powers that others want to control or use for their own gains.
A common plot element is the mystery surrounding the amnesia. Usually, the 'ofc' (original female character) has no memory of important events, and the vampires in the story are often involved in either causing or trying to reverse it. For example, a vampire might have compelled her to forget something crucial, and the story then unfolds as she tries to piece her past back together.
Well, you can try searching on fanfiction websites like Archive of Our Own. There are likely many stories with this theme there. Just type in the relevant keywords in the search bar and browse through the results.
Love is also a common theme. It could be romantic love for her character or platonic love within the family. In some fanfictions, Klaus's sister falls in love with a character from the show, and this love story is intertwined with her relationship with Klaus. For instance, in 'Love and Blood', her love interest is a werewolf, which creates a lot of drama considering the vampire - werewolf dynamic in the Vampire Diaries.
One interesting fanfiction could be where the Gilbert sister discovers a hidden power within herself that links her to the vampire world in a unique way. Maybe she has the ability to see the true nature of vampires, even when they are using their compulsion. This could lead to all sorts of interesting plot twists as she gets involved with the main characters of the show.
One common theme is her struggle to fit into the world that has been turned upside down by the presence of vampires. She often has to deal with the danger that vampires pose to her and her loved ones while also trying to understand their complex nature.