One plotline could be that there's a cupcake - making competition in their village. Sakura enters, with Calla as her mascot. Shisui, who is initially just a spectator, gets roped into helping Sakura when she faces some difficulties. They create a cupcake that has calla - like decorations on it and end up winning the competition, much to everyone's surprise.
Well, it could be a really cute story. Maybe Shisui and Sakura are in a cupcake shop and they see calla - shaped cupcakes. Shisui could be a bit of a prankster and try to steal a cupcake, while Sakura scolds him in a friendly way. And then they end up sharing a cupcake and having a great time.
One possible plotline could be that Itachi and Shisui, being the skilled Uchiha they are, take Sakura under their wing to train her in some unique jutsu. Maybe they see her potential and want to help her reach a new level of power. For example, Shisui could teach her his body - flicker technique while Itachi imparts some of his knowledge about genjutsu. This would create an interesting dynamic as Sakura tries to adapt to their different teaching styles.
One possible plotline could be that Shisui, with his unique abilities, takes Sakura under his wing to train her in some special jutsu. Maybe he sees her potential that others haven't fully recognized. He could teach her how to better use her chakra control in a more unique way than Tsunade taught her.
It could be that Shisui has been acting secretively lately. Itachi, being observant, and Sakura, who is also perceptive in her own way, notice this. They glare at him because they suspect he might be involved in something that could put the village at risk. Maybe he has been meeting with some suspicious characters. Itachi and Sakura's glares are a way of showing that they are on to him and that he should come clean about what he's been doing. As the story unfolds, Shisui might reveal that he was actually trying to protect the village in his own way, but his methods were misunderstood by Itachi and Sakura, leading to a lot of drama and tension in the fanfiction.
One plotline could be that Itachi, Sakura, and Shisui team up to protect a small village from a group of bandits. They use their respective skills, with Itachi's sharingan, Sakura's medical ninjutsu and combat skills, and Shisui's unique jutsu. Along the way, they learn to trust and rely on each other more deeply, forming a strong friendship.
A plotline could be that Shisui's spirit is somehow trapped and only Sakura, with her unique chakra control and medical knowledge, can sense and free him. As she tries to help him, they develop a connection and he starts to guide her in developing new ninja techniques, which also leads to a growing emotional bond between them.
Maybe Sakura is a healer in a small village. A gypsy arrives with a strange illness that no one in the village can cure. Sakura tries her best to help the gypsy, and in the process, she discovers that the gypsy has a unique connection to an ancient magic. This leads Sakura to learn more about this magic from the gypsy, which changes her life and the way she views the world around her.
Another plot could involve Naruto and Shisui going on missions together. Since Naruto is often underestimated in the original series, Shisui's presence as his brother might make others take him more seriously. They could form a powerful duo, with Naruto's brute strength and Shisui's Sharingan abilities complementing each other, and face off against powerful enemies like Orochimaru or Akatsuki members.
Another storyline could involve a dangerous situation on the train. Maybe there are bandits or some sort of enemy ninjas attacking the train. Shisui, being the powerful ninja he is, jumps into action. Sakura also uses her medical and combat skills to help. In the process, they form a bond of trust and friendship. After the incident, they continue to be friends and Sakura might even learn some new techniques from Shisui.
Well, it could be that the terrifying aspect of the fanfiction involves Sakura having some sort of dark power. Maybe she's turned evil in this fan - made story, and Shisui is shocked or scared by this unexpected turn of events.