One great 'Katherine Pierce Fanfic' is 'The Darker Side of Katherine'. It delves deep into her complex character, exploring her past and her relationships in a really interesting way. The author does a wonderful job of staying true to her personality as we know it from the show.
A good 'Katherine Pierce Fanfic' should first and foremost capture her essence. She's a complex, manipulative, and yet somewhat sympathetic character. So, the story should reflect that. For example, it could explore her reasons for her actions in the original series.
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.
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 fanfiction where Bella Swan becomes a Pierce Vampire, her appearance might be different. Pierce Vampires could have distinct physical features. Maybe her eyes turn a different color, or she has a more ethereal glow. Her style of dressing could also change to reflect her new vampiric status. She might be more drawn to darker, more elegant clothing. Her interactions with other characters would be fascinating to explore. How would the Volturi react to her transformation? Would they see her as a threat or an ally? And how would her human friends, like Jessica and Angela, perceive her now that she's a different kind of vampire?
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.
Maybe Bella begins to idolize Katherine so much that she tries to emulate her style and behavior. This could lead to a rift between her and Edward, as he doesn't understand her newfound fascination. Bella might then get involved with some of Katherine's old enemies or allies, unknowingly bringing a whole new set of problems to Forks. It could also involve her trying to learn some of Katherine's vampire - related skills, like compulsion, if that was possible in the context of the story.
It really depends on the perspective. Elena is often seen as the good girl, the one with a pure heart. She represents the moral compass in many fanfictions. But Katherine has this allure of mystery and a long history of survival. If you like characters with depth and a darker side, Katherine might be better in the fanfiction world. However, if you prefer the more traditional heroines, Elena would be the top choice.
Her relationship with Edward would definitely change. Edward is used to a more innocent and reserved Bella. If she becomes a Katherine Pierce fan, she might become more assertive and independent, which could either make Edward more attracted to her in a new way or cause him to be wary. He might not like the new, more dangerous side of Bella that is emerging.