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.
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.
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.
The OC should have flaws as well as strengths. For instance, they might be overly impulsive, which often gets them into trouble but also makes them brave in the face of danger. Their backstory could involve being part of a secret society that was wiped out by Katherine, giving them a personal vendetta. In terms of appearance, give them a signature style of dressing, like always wearing a leather jacket with a specific symbol on it. This makes them stand out. And when it comes to their dialogue, make it full of spunk and determination, showing their strong - willed nature.
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.
In my opinion, if the OC Salvatore has been training specifically to take on Katherine and has some sort of unique advantage, like immunity to her mind - control tricks, then the OC might win. However, Katherine is not one to go down easily. She has survived for centuries by being smart and adaptable. If the OC makes a single mistake, Katherine could turn the tables on them. So it's really a toss - up between their powers, skills, and the situation in the fanfic.
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.
One great Damon Salvatore OC fanfiction is 'A New Beginning'. It has a really interesting original character who brings a fresh dynamic to Damon's world. The plot is engaging as it shows how the OC deals with the complex vampire world Damon is part of.
There's also 'Salvatore's Naruto Adventure'. In this fanfic, Naruto is somehow transported to the Salvatore world. He has to adapt quickly, and his chakra abilities make him a very different kind of presence in that world. The fanfic often focuses on how he interacts with the existing characters there and how his presence changes the power dynamics.
There's this fanfiction called 'Hope and the Mysterious OC'. In it, the OC character has a unique backstory that ties in well with the world of Hope Mikaelson and the Salvatores. It's full of mystery and some great character interactions. The author does a wonderful job of developing the relationships between all the characters, making it a must - read for fans of this crossover idea.