One idea could be a story where Sakura, as a skilled medic, is tasked with treating Itachi and Shisui during a difficult mission. She discovers their complex pasts and bonds with them over time, leading to new alliances and friendships.
To write a good fanfic, focus on character development. Let Sakura grow in her abilities and relationships. Show how Itachi's past affects his present actions when interacting with Sakura and Shisui. For Shisui, explore his loyalty and how that impacts the group dynamic. Also, use vivid descriptions of the Naruto world, like the Hidden Leaf Village or the battle scenes. This will make the fanfic more immersive.
One possible plot could be a story where Sakura, Itachi, and Shisui are all part of the ANBU. Itachi and Shisui could be mentoring Sakura in the ways of the ANBU. They might go on a dangerous mission together, facing powerful enemies. Shisui could use his unique abilities like Kotoamatsukami to gain an advantage, while Itachi's Sharingan and combat skills are also crucial. Sakura, with her medical ninjutsu and growing combat skills, could prove to be a valuable asset in the team.
Sakura would likely be in awe of Itachi and Shisui's skills at first. Itachi, being the more reserved one, might be a bit distant but would gradually warm up to Sakura as he sees her determination. Shisui, on the other hand, would be more friendly and approachable. He might be the one to initiate conversations and break the ice between the three. They could have exchanges about their different ninja backgrounds.
Well, one possibility could be that they are on a mission together. Sakura's medical skills could come in handy when Itachi or Shisui get injured. And they might face a powerful enemy that requires all of their skills combined to defeat. Maybe Itachi's Sharingan and Shisui's unique abilities, along with Sakura's strength and intelligence, make for an interesting dynamic.
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.
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.
Sakura would likely develop her combat skills further. In the ANBU environment, she would face more dangerous opponents, which would force her to become more strategic. Itachi, already a complex character, might show more of his internal struggle between his loyalty to the village and his love for his family. Shisui could develop in terms of his leadership skills, as he and Itachi might lead Sakura in missions. He could also become more jaded as he faces the darker side of the ninja world.