On the other hand, Sakura's amusement might also act as a buffer. Itachi might be less likely to get truly angry at Shisui if he sees that Sakura is enjoying the situation. It could turn their relationship into more of a light - hearted, friendly rivalry with Sakura as an onlooker. For example, if Shisui and Itachi were to have an argument over Shisui's annoying behavior, Sakura's laughter might defuse the tension and make them realize it's not that serious.
Sakura's amusement could actually make Shisui more likely to continue annoying Itachi. Since she finds it funny, Shisui might see it as a sign that his actions are acceptable and entertaining. This could potentially strain the relationship between Shisui and Itachi further if Itachi doesn't like being the target of Shisui's annoyance.
Well, perhaps the plot could involve Shisui constantly playing pranks on Itachi, which Sakura finds hilarious. Maybe Shisui hides Itachi's training tools, and Itachi's confused and flustered reactions make Sakura giggle. It could also be that Shisui teases Itachi about his secret crush, if he has one, and Sakura enjoys seeing Itachi being so out of his element.
Well, perhaps Sakura is in a really bad mood and Shisui is just in the wrong place at the wrong time. She starts pummeling him and Itachi, who has a rather dry sense of humor, is amused by the whole situation. It could also be that Shisui has been teasing Sakura a lot and she finally snaps and goes after him, with Itachi looking on with that amused look on his face.
The main characters are clearly Itachi, Sakura and Shisui. Itachi is the one who is amused, Sakura is the one doing the pummeling and Shisui is on the receiving end of Sakura's attack.
Perhaps Sakura is amused by Itachi's unique skills or his way of handling things. Meanwhile, Shisui annoys her because he has a different approach to things that she doesn't like. In the fanfiction, it could be that Shisui is too brash compared to Itachi's calmness, and Sakura, being a fan of Itachi, just can't stand Shisui's antics.
A possible plot could be that Itachi is involved in a political intrigue within their ninja village. Sakura, being a bystander but a fan of Itachi, is observing the situation. Shisui, believing he can help but actually making things worse, keeps butting in and causing chaos. His actions are annoying to everyone, especially Sakura. However, Sakura is also amused by how Itachi gracefully navigates through the mess created by Shisui, always staying one step ahead and maintaining his dignity.
There could also be a more friendly and equal - footing relationship. They might bond over shared interests outside of combat, like a love for a certain type of flower in the village or a particular kind of food. This would bring them closer together on a personal level. Itachi, who is usually reserved, might open up more around Sakura and Shisui, and they could form a tight - knit group of friends who support each other through the difficulties in the shinobi world.
Sakura's character could develop in terms of her understanding of the ninja world. Itachi and Shisui have different outlooks compared to her. Through training with them, she might gain new perspectives on things like loyalty, sacrifice, and the greater good in the ninja world, which in turn shapes her character.
There are quite a few good writers out there. One way to find them is to search for fanfiction communities dedicated to Naruto (since these characters are from Naruto). In these communities, people often recommend their favorite writers. Some writers are known for their in - depth characterizations of Itachi, Sakura and Shisui. They make the characters feel real and add new dimensions to their relationships. You might also want to follow some fanfiction blogs on Tumblr that often share and promote great fanfictions about these characters.
One possible fanfiction could be about Sakura joining an Anbu team that has Itachi and Shisui. Maybe Sakura is a new recruit and she has to learn from the two experienced shinobi. Itachi could be the calm and collected leader, while Shisui is the more easy - going one who helps Sakura adjust to the harsh Anbu lifestyle.
A common theme is time - travel. For instance, Sakura might travel back in time and meet a younger Itachi and Shisui. This allows for interesting plot developments as she tries to change the future or just understand their past better. Also, the theme of redemption can be found. Maybe Itachi or Shisui have made some mistakes in their past and Sakura helps them find redemption through their interactions with her.