Perhaps Sasuke has been influenced by some dark force. This force could be manipulating him into believing that Naruto is a threat to his goals. So, he kidnaps Naruto to either stop him from interfering or to convert him to his side. It could lead to a very intense struggle within Naruto to resist Sasuke's influence.
One possible plot could be that Sasuke has a hidden agenda. Maybe he believes Naruto is in danger from a new, unseen enemy in the ninja world. So, instead of explaining his reasons to Naruto, he kidnaps him to keep him safe in a secret location. However, this action might be misinterpreted by their friends, leading to a search and rescue mission.
If Sasuke kidnapped Naruto, it could initially seem like a huge betrayal. However, Naruto is known for his ability to forgive. He might see this as Sasuke still caring about him in his own, twisted way. This could make their relationship more complex, with Naruto constantly trying to reach out to Sasuke and Sasuke struggling with his own emotions and reasons for the kidnapping.
Sasuke would likely be enraged. Naruto is his rival and also, in a way, his friend. He would immediately set out to find Naruto and confront Itachi. Sasuke has a strong sense of pride and would not let his brother get away with this.
Well, Naruto might be furious. He's always been protective of Sakura. He'd probably drop everything he's doing and go after Sasuke to rescue Sakura. Naruto has a strong sense of justice and loyalty, especially towards his friends. And Sakura is one of his closest friends, so he won't stand by and let her be in danger.
She would probably be very angry at first. Naruto is a headstrong character, and being kidnapped against her will would make her furious. She'd likely try to fight back immediately, using whatever skills she has at her disposal.
The kidnapped character could use their own hidden jutsu skills to break free. For example, if they have a teleportation jutsu they've been hiding, they could use it when Sasuke is distracted. Another way is to appeal to Sasuke's emotions. If they can remind him of his better nature or a past event that made him more empathetic, he might let them go.
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.
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.
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.