Another plot could be that the Salvatore sister is a key figure in an ancient prophecy. The vampires, werewolves, and witches all believe she can either bring great destruction or salvation to their world. This would put her in the middle of a power struggle, and her brothers would have to fight not only to protect her but also to ensure she makes the right choice for the greater good.
Perhaps the Salvatore sister was turned into a vampire long before her brothers, and she has been in hiding all this time. When she finally reveals herself, she has a whole different set of vampire ethics compared to them. She could teach them new ways of co - existing with humans without causing too much harm, which would lead to a lot of interesting interactions within the Mystic Falls community.
Perhaps the sister has a special power that she's not aware of, and the sleeping spell was cast to keep that power dormant. In the fanfiction, a vampire who has a soft spot for the sister might try to wake her up, but in doing so, risks awakening her power which could be either a great asset or a huge danger to the town. They could face challenges like having to deal with the witch who cast the spell in the first place, who doesn't want the power to be unleashed.
A common plotline is the Salvatore sister coming back to life after being thought dead for a long time. This brings a lot of drama as her brothers have to adjust to having her around again. Also, she might be a hybrid, part vampire and part something else. This gives her unique abilities and makes her a target for different factions in the story. Many fanfictions explore how she deals with her new identity and the powers that come with it.
One plot idea could be that the OC sister discovers her vampire siblings have been secretly using her blood for their own gain. This betrayal could lead to her seeking revenge. She might team up with other supernatural beings in Mystic Falls to get back at them.
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 plot idea could be that the sister, after being drugged, has some sort of psychic connection with the vampires in The Vampire Diaries world. Maybe she can see their memories or feel their emotions, which leads to a series of adventures where she tries to use this new - found connection to help or hinder the vampires' actions.
A human could also be a love interest. The Salvatore sister might be intrigued by the simplicity and innocence of a human life. This relationship would be complex as she has to hide her true nature from the human, and also protect him from the other supernatural beings in Mystic Falls.
Another plot idea is that the Salvatore sister is forced to team up with Katherine. Perhaps there's a new, powerful enemy in town that only their combined knowledge and powers can defeat. As they work together, the sister starts to see Katherine in a different light, not just as the villain the Salvatores always thought she was. This could create a complex relationship full of tension and unexpected alliances.
One idea could be being reborn as a long - lost relative of the Salvatore brothers. Maybe you have some hidden power that was lost in the family history and now you have to navigate the vampire - filled world of Mystic Falls while trying to figure out your new abilities. Another plot could involve being reborn as a witch in the fanfiction world. You could use your magic to either help or hinder the vampires and other supernatural beings in town. You might also be reborn as a human with a special gift of seeing the future, which would put you in a dangerous position as everyone in the Vampire Diaries world would want to use your power for their own ends.
One idea could be being reborn as a long - lost relative of one of the main characters. For example, a long - lost sister of Elena. This could stir up a whole new dynamic in the Mystic Falls gang. You could bring in new powers or a different perspective on the vampire - werewolf - witch drama.
One plot idea could be that Mason, instead of being killed, manages to escape from Klaus. He then teams up with Elena to find a way to permanently defeat Klaus. They go on a journey to find ancient artifacts that can be used against the original vampire.