It could be the key to solving a major mystery in the story.
The jewel might make Sakura a target, which would create a lot of action and drama. Enemies could be after it, forcing her to defend herself and the jewel. This would lead to battles and tense situations. Also, it could be that her friends are initially suspicious of the jewel, which would cause some internal conflict within the group until they understand its true nature.
In many fanfics, such a jewel could be a magical or mystical element. It could be a source of her strength, like giving her the ability to perform special jutsu or have enhanced senses. It might also have a connection to her relationship with Sasuke. For example, it could be a gift from him in the past or something that binds them together in a special way.
If Sakura dies in a fanfic, it could have a huge impact on the whole Naruto -verse within that story. Her friends like Naruto would be deeply affected. Naruto might feel like he failed to protect her, just as he felt about other losses in his life. And for the medical ninja community in the fanfic, her death could be a big blow as she was a talented medic.
A fanfic could show Sakura sacrificing herself to save Sasuke. This would be a very emotional and heroic end for her. Sasuke would then have to live with the guilt and the memory of her selfless act.
It might explore different aspects of Sasuke and Sakura's characters within the context of a fanfic. For example, it could show Sakura with different abilities or Sasuke having a different attitude towards her. Since it's a fanfic, the author has the freedom to deviate from the original storyline and create something new. It could also be about their relationship in a different time period or setting, like in a modern - day AU (alternate universe) or in a completely fictional world.
I'm not entirely sure as I haven't read this specific fanfic. But generally, in SasuSaku (Sasuke and Sakura from Naruto) fanfics, it could be about some new adventure that breaks Sakura's seal and how that impacts her relationship with Sasuke and the overall story world.
It could be Orochimaru. He's always been interested in Sasuke and might see kidnapping Sakura as a way to manipulate Sasuke. He could use Sakura as a guinea pig for his experiments or as a bargaining chip to get Sasuke to come to him.
Well, one possibility could be that it's a dark fanfic where an enemy of Sakura kidnaps her to get to Sasuke. Maybe they want to use her as bait to lure Sasuke into a trap. Since Sasusaku is a popular pairing, the kidnapper might think that Sasuke would do anything to save Sakura.
In some cases, Karin can be more popular. Her backstory of being associated with Orochimaru and her survival instincts make her an appealing character. Some fans might be drawn to the idea of her as an underdog in the love triangle with Sasuke and Sakura. Also, her sensing abilities and her role in certain plot points in the original series can be explored more deeply in fanfics, which might make her more interesting to some readers than Sakura.
You can try some popular fanfic websites like FanFiction.net or Archive of Our Own (AO3). These platforms usually have a large collection of Naruto - related fanfics, including sasusaku ones with an evil Sakura theme.
One popular sasusaku evil sakura fanfic might be 'The Darkened Sakura'. In this story, Sakura's character takes a dark turn perhaps due to some unforeseen circumstances in the Naruto universe. Another could be 'Evil Sakura's Ascent' which shows how Sakura uses her skills in a more malevolent way. There's also 'Sakura's Shadow Self' where an alternate, evil version of Sakura emerges and causes chaos in the village.