It could be Orochimaru. He's always had an interest in powerful shinobi, and Sakura has shown great potential. Itachi might be roping Sakura's wrists to prevent Orochimaru from getting to her and using her for his evil experiments.
There's also the possibility that the main antagonist is an original character created for this fanfiction. This new character could have a vendetta against Sakura or be interested in the power she holds, and Itachi is trying to stop them by restraining Sakura in this way.
Madara Uchiha could also be a likely antagonist. His grand plans often involve manipulating various characters. If he sees Sakura as a threat to his plans, Itachi might be taking preemptive action by restraining her, which could put him in a conflict with Madara.
It could be a story where Itachi has some sort of ulterior motive for restraining Sakura. Maybe he's trying to protect her in a very unorthodox way. For example, he might be aware of a danger that only he can sense, and by restraining her wrists, he's ensuring she doesn't run into harm's way.
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.
Itachi might be doing this as a means of restraint to prevent Sakura from interfering in a dangerous situation. He may know that she has a tendency to rush into things without thinking, and this is his way of ensuring she stays put. Another idea is that it could be related to a cultural or ritualistic practice within the context of the fanfiction's world.
Sakura could also be confused. She may try to reason with Itachi, asking him why he is doing this. If she trusts him to some extent, she might be more compliant while still expressing her discomfort. She may start to analyze the situation, trying to figure out if there is a hidden meaning or purpose behind his actions.
Itachi himself could be seen as the main antagonist in this scenario as he is the one performing the kidnapping. His actions are against Sakura's will and are likely to cause distress and conflict.
Fanfiction is fictional stories written by fans. In this case, it might be about a scenario where Itachi pins Sakura's wrists. It could be part of an action - filled, or perhaps a more complex and dramatic story within the fan - created world of the relevant anime or manga.
A rogue ninja who wants to steal the power of the contract for his own evil purposes could be the main antagonist. He might be jealous of the connection between Sakura and Itachi and wants to break it to gain the abilities they could potentially unlock together.
I'm not sure if it's a particularly popular type. There are so many different kinds of Naruto fanfiction out there, focusing on various relationships and scenarios. While the idea of Itachi and Sakura interacting in this way might be interesting to some, it may not be as widespread as other more common pairings or plotlines.
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.
Itachi himself could be seen as the main antagonist in this scenario. After all, he is the one doing the kidnapping.