It could be Orochimaru. He has always been interested in Sasuke and might kidnap Sakura to use her as bait to get Sasuke back under his control. He could be after Sasuke's power and think that by using Sakura, he can manipulate Sasuke easily.
Another possibility is a group of anti - Konoha militants. They view Sakura as an important asset to Konoha because of her medical skills. By kidnapping her, they hope to weaken Konoha's medical support system and also use her as a bargaining chip against Sasuke, who they see as a threat to their anti - Konoha plans.
One possible fanfic could start with Sakura out on a solo mission in a forest. Sasuke, who has been misled by some dark force, sees this as an opportunity to get Sakura for some unknown reason. He uses his ninja skills to quickly subdue her and takes her to a hidden cave. Sakura is shocked and tries to reason with him, but Sasuke seems to be in a trance. Meanwhile, Naruto and the others start to search for her when they realize she's missing.
A possible plot for a 'Sasuke Sakura Kidnapped Fanfic' could be that a mysterious organization kidnaps Sakura. This organization is secretly researching the Sharingan and believes that Sakura, due to her close relationship with Sasuke, has some knowledge about it that they can use. Sasuke, of course, is enraged when he finds out. He embarks on a solo mission at first, but later realizes he needs the help of his friends. Along the way, we can explore the dynamics between Sasuke and Sakura, like how Sasuke regrets not being there to protect her, and how Sakura remains strong in the face of danger, believing Sasuke will come for her.
Sakura could be kidnapped by Sasuke when she's on her way to deliver some important medical supplies. Sasuke, who is on a self - imposed mission that he believes requires Sakura's presence, takes her. For example, he might think she can heal a certain injury that he has. Sakura, being the strong - willed character she is, tries to escape at every opportunity during their journey to his hideout.
A rogue shinobi who has a personal vendetta against Sakura or her friends. Maybe this shinobi blames Sakura for something that happened in the past and decides to kidnap her as revenge. He could be a powerful and unpredictable antagonist.
Sakura could use her medical ninjutsu skills to create a distraction. For instance, she might make it seem like she's poisoned herself, causing Sasuke (or whoever is involved in the kidnapping) to get closer, and then she could attack and escape.
Sakura would probably be initially shocked and angry. She has always cared about Sasuke, and this betrayal would hit her hard. But she is also a strong - willed ninja. So, she would quickly gather her wits and start looking for ways to escape. She might try to use her knowledge of Sasuke's fighting style against him if she gets the chance.
In many Sasuke - Sakura fanfictions, a kidnapping plot can add a lot of drama. If Sasuke kidnapped Sakura, it could be due to various reasons. Maybe he was influenced by some dark power that made him lose his rational thinking and see Sakura as a key to something he wants. This kind of plot often leads to a series of events where Sakura has to face her feelings towards Sasuke in a very extreme situation, like trying to understand his motives while also being scared for her own safety.
Kakuzu might also be a main antagonist in this fanfiction. He's always after money and power. Maybe he sees Sakura as a valuable asset, either to sell her back to Konoha for a large sum or to use her skills for his own gain. He's not very empathetic and would likely be ruthless in his treatment of her.
It's possible that such a fanfic exists. The world of fanfiction is full of all kinds of creative and sometimes unexpected interpretations. Some writers might want to break the mold and show Sakura and Sasuke in a less than ideal light to explore different storylines or character developments. But it's not something you'd find easily among the more popular fanfics.
There could be. The world of fanfic is vast and diverse. Some writers might create stories where Sakura cheats on Sasuke to add drama, explore different character dynamics, or simply as an alternative universe scenario. But it's important to note that this is not in line with the main Naruto narrative.