Well, one possibility could be that Itachi is under some sort of mind - control. Maybe an enemy has forced him to target Sakura as a way to get to Naruto or the rest of Team 7. He holds the kunai to her throat but is fighting internally against the control, his eyes showing the struggle.
Perhaps Itachi is trying to protect Sakura in a very strange way. He knows that there are some hidden threats around. By holding the kunai to her throat, he is making it seem like he is the danger to her, when in fact he is drawing out the real enemies who want to harm Sakura. He has a plan that no one else is aware of, and Sakura has to trust him despite the very intimidating situation.
Sakura would probably be scared at first. But then, being the strong - willed character she is, she might start to talk back to Itachi. She could say something like 'You don't want to do this, Itachi. I know you have your reasons, but this is wrong.' She would try to use her words to defuse the situation.
Perhaps in this fanfiction, Itachi has a certain 'weight' of expectations or a mission that somehow involves Sakura. She might be struggling to deal with his decisions or the way he's affecting the situation around her. For example, if Itachi is on a path that goes against Sakura's beliefs, she could be fighting internally on how to react or whether to try and change his course. Another possibility is that Itachi has some form of power or ability that is creating a difficult environment for Sakura to operate in, and she is struggling to adapt or overcome it.
Maybe Itachi, despite his stoic nature, starts to have an unexpected soft spot for Sakura. He could be secretly protecting her during her missions, which might lead to some tense and mysterious moments as Sakura tries to figure out who her mysterious protector is.
Well, perhaps the plot could involve Sakura being assigned as Itachi's secretary in a powerful ninja organization. At first, Itachi is cold and distant towards her, but Sakura is determined to do her job well. As time passes, they start to have mutual respect for each other's skills. Then, an external threat to the organization emerges, and Sakura and Itachi have to work together closely, which leads to a deeper understanding and maybe even the start of a romantic relationship between them.
Another plot twist could be that Itachi is under some sort of mind - control or illusion when he abducts Sakura. This could lead to a scenario where Sakura has to find a way to break the control on Itachi and in the process, they form an unexpected alliance. It would add an element of mystery and a chance for character development for both of them.
Perhaps in the fanfiction, Sakura and Itachi are in a police force that has to deal with international ninja disputes. Sakura, with her intelligence and strong will, and Itachi, with his strategic mind, work together to prevent a full - scale war between two large ninja clans. They face many challenges along the way, including distrust from their own comrades who are skeptical of their unorthodox methods.
The plot could involve Itachi noticing Sakura's isolation within her own team at some point. He decides to adopt her in a sense, like becoming a mentor to her. Itachi could share his insights on how to deal with powerful enemies and the sacrifices one has to make for the greater good. Sakura would then grow not only in her combat skills but also in her understanding of the complex world of ninjas. She might start to see things from a different perspective, similar to Itachi's, which could cause some friction and also growth within her relationships with Naruto and Sasuke.
Perhaps Sakura gets into trouble during a training exercise and Itachi has to rescue her, which then leads to a more intense and focused training regime. Another development could be that Sakura starts to think like Itachi in battle situations, using his tactics to outwit her opponents. It could also be that as she gets stronger, other characters start to notice and either become jealous or try to recruit her for their own purposes.
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.
One possible plot could be that Sakura, with her new family connection to Itachi, gets trained by him in secret. This would make her much more powerful and give her an edge in the ninja world. It could also lead to her being targeted by the same enemies as Itachi.