A possible plot could involve the Salvatore sister being a reincarnation of someone from Katherine's past. Katherine initially mistakes her for this old acquaintance and tries to manipulate her. But as time goes on, the sister shows her own unique personality and strength, and they end up in a power struggle. Along the way, the sister might also learn some dark secrets about her own past that are linked to Katherine's actions in the past.
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.
Well, the OC could be a vampire created by Katherine herself long ago but forgotten. When they meet again, the OC is torn between loyalty to Katherine and a newfound sense of right and wrong influenced by the Salvatores. Maybe the OC has the ability to read minds, which makes them a valuable asset in the battle against Katherine as they can anticipate her moves. Also, the OC could be from a different dimension where the rules of vampirism are different, and they bring this new knowledge to deal with Katherine, who is intrigued by their other - worldly powers.
One idea could be that the OC Salvatore has a unique power, like the ability to control time briefly. They use this power to outwit Katherine Pierce in a battle of wits and strength. Maybe they are fighting over a powerful magical artifact that Katherine wants to use for her own evil purposes.
A common theme is love triangles. Since Katherine has a history with the Salvatores, an OC as their sister can easily get caught up in the complex web of emotions. For example, she might be in love with someone one of the Salvatores also has feelings for.
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.
Well, Archive of Our Own (AO3) is also a great source. It has a large collection of fanfics. To find 'oc salvatore sister katherine fanfic', you might start by typing in the full phrase in the search bar. Then, look through the summaries and tags of the stories that come up. Some might be about an original character who is related to the Salvatores and has some sort of connection with Katherine, whether it's a rivalry or an alliance.
The relationship could start with hostility. Since Katherine has a history with the Salvatores, the sister might be naturally distrustful. But then, through a series of events like facing a common threat together, they could gradually develop a grudging respect for each other. For example, if they both get trapped by a powerful vampire hunter, they have to rely on each other's skills to escape.
One idea could be a story where Katherine and Elena have to team up to fight a new, powerful supernatural threat in Mystic Falls. They put aside their differences and use their combined skills and knowledge to outwit the enemy.
One idea could be that the OC is a powerful witch who gets caught in the middle of the Salvatore - Rebekah drama. She might initially befriend Rebekah, not knowing her full vampire nature, and then fall for one of the Salvatores. This creates a love triangle full of supernatural twists.
One idea could be that they face challenges from the supernatural community due to their unborn child's potential power. The other vampires or witches might see the baby as a threat or a valuable asset. Another plot could be them preparing for the arrival in a world full of danger, like building a safe haven and gathering allies.
One idea could be that the baby has some sort of special power. Since Elijah is a powerful vampire and Katherine has her own unique charm and skills, their baby might inherit something extraordinary. Maybe the baby can control the minds of vampires or has an uncanny ability to sense danger.