It could be Naruto. Naruto has a strong bond with Sakura and would sense that something was wrong. He'd probably track her down using his amazing sense of smell or by asking around. Once he found Sasori, Naruto would use his Rasengan and his newfound Sage Mode powers to take on Sasori and rescue Sakura.
Another possibility is Kakashi. Kakashi is Sakura's former teacher and is very protective of his students. He would notice Sakura's disappearance quickly. Using his Sharingan and his vast combat experience, he would strategize a plan to infiltrate wherever Sasori is holding Sakura and rescue her.
Well, in such a fanfiction, Sasori might be stalking Sakura to find the perfect moment to kidnap her. Maybe he has some ulterior motives related to her medical knowledge or her connection to certain powerful shinobi. He could use his puppet skills to ambush and take her away quietly.
Obviously, the main characters are Sasori and Sakura. Sasori is the one doing the stalking, and Sakura is the target of his stalking.
The plot of 'sasori stalks sakura fanfiction' might involve Sasori stalking Sakura as part of a larger plan. For example, he could be working for a more powerful villain who has an interest in Sakura. Sasori is sent to observe her, her daily routines, her relationships with other ninjas. As he stalks her, he might start to develop some sort of internal conflict. On one hand, he has his mission, but on the other, he may start to respect or even have a strange liking for Sakura, which makes the whole situation more complex.
Well, if Sasori kidnaps Sakura in fanfiction, it could lead to a really intense situation. Sasori is a formidable opponent. Sakura, on the other hand, has her medical ninjutsu skills and her growing strength in combat. She could potentially use her medical knowledge to her advantage. For example, she might be able to use some herbs or substances in the place where she's held captive to create a distraction or even a weapon. And there could be a rescue mission by her friends from Konoha, which would add more drama to the story.
Sasuke might also be involved in the rescue. Despite their complicated relationship, he still has some feelings for Sakura. And he wouldn't want to see her in the hands of someone like Deidara. Also, Kakashi, as their former sensei, would likely lead a rescue mission. He has the skills and experience to deal with Deidara's explosive attacks and find a way to save Sakura.
Sakura could use her medical ninjutsu to create an opening. For example, she might be able to use a numbing agent on Sasori or his puppets to slow them down and then make a run for it.
Naruto would surely be one of the first to try and rescue Sakura. He has always been protective of her and would not stand by while she's in danger.
Itachi himself could be seen as the main antagonist in this scenario as he is the one carrying out the kidnapping. He is acting against Sakura's will and causing a dangerous situation for her.
Naruto could be the one to save her. He has a strong bond with Sakura and would never let her be in danger for long. His powerful ninja skills and his unwavering determination would make him a likely candidate to rescue her from Black Zetsu's grip.
Well, in fanfiction, Sasori and Sakura can be put into various scenarios. Since Sasori was once an antagonist, it's interesting to think about how Sakura, who represents the side of good, would interact with him. If they are forced together, it could be like a clash of two different worlds. Sasori is so used to his own way of doing things with his puppets, and Sakura is used to her ninja training and teamwork with her comrades.