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.
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.
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.
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.
In this fanfiction scenario, it could be really interesting. Shisui's amusement might add a unique dynamic. He could be observing Sakura snuggling Sasuke and have various thoughts. Maybe he's secretly rooting for their relationship, or perhaps he finds it a bit comical considering their different personalities. Sasuke, who is usually so stoic, having Sakura snuggle him could be a big step in their relationship development within the fanfiction world.
One possibility is that other characters will get involved. For instance, Naruto might see this and be a bit surprised or happy for his friend Sasuke. Also, the fact that Shisui is amused could spread around the village, leading to different reactions from the villagers. Some might support the relationship, while others might be against it. This could create some conflict for Sakura and Sasuke to overcome in the fanfiction.
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.