Sakura's medical skills and Shisui's combat skills can be combined in a battle scenario. For example, if Shisui gets injured during a fight, Sakura can quickly use her medical ninjutsu to heal him so he can get back into the fight. Or, Sakura can use her knowledge of the human body to help Shisui target his opponents' weak points more effectively. Shisui, in turn, can use his Sharingan to detect incoming attacks and warn Sakura, allowing her to better protect herself while she is treating the wounded on the battlefield.
Sakura's ability to perform complex surgeries and her understanding of the human body can be paired with Shisui's Sharingan - enhanced perception. Let's say there is a situation where a powerful ninja has been poisoned and is on the verge of death. Sakura needs to perform a dangerous operation to remove the poison. Shisui can use his Sharingan to monitor the patient's vital signs and the flow of chakra during the operation. He can also use it to detect any possible threats or enemies approaching the operating area. His combat skills can be used to fend off any attackers while Sakura focuses on the surgery. Also, Shisui's experience in espionage could be combined with Sakura's intelligence gathering skills as a medic - ninja. They could work together to uncover a conspiracy within the ninja village by using their unique skillsets.
You can start by looking on fanfiction platforms like FanFiction.net or Archive of Our Own. Just type in 'sakura shisui' in the search bar and a whole list of relevant fanfictions will pop up.
Sakura and Shisui in a fanfiction as lovers might have a relationship that is full of challenges. Given their different backgrounds and the complex world of the Naruto series. Shisui being an Uchiha has his clan's issues to deal with, while Sakura is focused on her growth as a ninja. However, their love overcomes these obstacles. They support each other in their respective pursuits. For instance, Shisui helps Sakura train in combat when she wants to improve her physical skills, and Sakura uses her medical knowledge to keep Shisui in top shape during tough missions.
Sakura might become more reserved in an older time period as the social norms were different. She is used to the more modern and open ninja world. Shisui, on the other hand, might feel more at home initially as he is from a more traditional Uchiha background. But he could also be frustrated by the lack of modern conveniences and more rigid social structures.
Itachi's personality is one of self - sacrifice and loyalty. If he is glaring at Shisui, it could be because he believes Shisui is straying from the path of loyalty to the clan or the village. His glare would be a cold and serious one, making Shisui aware of the gravity of the situation. Sakura, who has grown from a love - struck girl to a strong - willed kunoichi, would bring her determination to the table. Her glare might be accompanied by some verbal challenges, as she is not afraid to speak her mind. The combination of Itachi's silent but powerful glare and Sakura's more vocal and emotional one would create a very tense atmosphere around Shisui.
Maybe he starts by observing Sakura's current chakra control. He might notice her flaws and then begin with basic chakra exercises to improve her stability.
In fanfiction, Sakura could be given the power of the Death Note. But instead of using it for evil like some in the Death Note story, she uses it to heal. She might find a way to transfer the 'death' aspect of the note into a 'healing' power, using her medical knowledge and chakra abilities.
Shisui would likely be very calm and observant. He might interact with Sakura in a somewhat reserved way at first. For example, he could just give her a polite nod when they first meet on the train. Sakura, being more outgoing, would probably try to start a conversation. She might ask him about his travels or his skills. Shisui would then answer in a measured tone, sharing only what he thinks is appropriate.
Their relationship could start with mutual respect. Shisui might respect Sakura's determination and her growth as a ninja. Sakura, on the other hand, would be in awe of Shisui's Sharingan abilities. As they go on missions together, they start to trust each other more and more.
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.
Sakura's medical ninjutsu skills would likely be more refined. Since Rin was an experienced medical ninja, she could teach Sakura advanced techniques early on. Sakura might also be more proficient in stealth and perception skills as Rin could have passed on her own knowledge in those areas.