A rogue ninja from a rival village could be the main antagonist. He might be after the secrets of the Sharingan that Itachi and Shisui possess. And he sees Sakura as an obstacle when she tries to protect her comrades. He could have some powerful and forbidden jutsu that makes him a formidable foe for the trio.
Maybe a powerful and ancient demon that has been awakened. It targets the ANBU because it wants to spread chaos and destruction. Itachi, Shisui and Sakura are the ones who stand in its way. The demon uses illusions and dark magic, which forces the three of them to combine their skills and find new ways to fight this supernatural foe.
It could be a corrupt ANBU official. This person wants to use Itachi and Shisui's power for his own evil plans. He sees Sakura as a threat to his schemes as she is known for her strong moral compass. He tries to frame them for various crimes within the ANBU to get rid of them and continue with his underhanded activities.
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.
One possible plot could be that Itachi and Shisui are on a mission in the ANBU and they cross paths with Sakura who is also on a mission related to a medical emergency in a war - torn area. They initially have some misunderstandings as Sakura is not used to the ANBU's way of doing things, but eventually they work together. Itachi and Shisui use their combat skills while Sakura uses her medical ninjutsu to save a village from a hidden enemy's attack.
Shisui is known for his cheerful and friendly nature. He would likely be the one to break the ice with Sakura and make her feel welcome in the Anbu team. Itachi, on the other hand, is more reserved. He would respect Sakura's skills but might not be as openly friendly at first. Sakura, being strong - willed, would earn their respect over time with her determination and medical knowledge.
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.
In the fanfiction, there could be a conflict within the team. Maybe there is a misunderstanding between Itachi and Shisui regarding a mission strategy. Sakura, being the mediator, tries to resolve the issue. She uses her understanding of both of them to bring them back together. This not only strengthens the bond within the team but also makes them more effective in future missions.
The main characters are obviously Itachi and Sakura. Itachi is a complex character from the Uchiha clan known for his powerful Sharingan abilities. Sakura, on the other hand, is a skilled kunoichi who has grown in her medical and combat abilities. In this fanfiction, they are usually placed in the context of the ANBU, which is a special and secretive part of the ninja world.
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.
One idea could be that Sakura, while on a mission with the Anbu team led by Itachi and Shisui, discovers a hidden jutsu that could change the balance of power in the ninja world. The Anbu team then has to protect her as she tries to master it and keep it from falling into the wrong hands.
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.
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.