It could be about Sakura having a strong admiration or being a fan within the Uchiha context in the fan - made story. Maybe she is a big fan of an Uchiha character like Sasuke, and the fanfiction explores her feelings, actions related to that admiration.
I'm not sure specifically what that particular 'sakura oldest uchiha fanfiction' is about as there could be many out there. It might be a fan - created story that involves Sakura from the Naruto series and an Uchiha character, perhaps exploring their relationship, or it could be about Sakura's connection to the history of the Uchiha clan in a fictional scenario.
There may be a vast amount of fanfiction out there, but this particular description involves inappropriate and vulgar content. Most fanfiction platforms have rules against such content. So, it's improbable that there exists a fanfiction with this description. Fanfiction usually aims to explore characters' relationships, adventures, and development in a more appropriate and respectful way.
One common theme could be Sakura trying to fit into the Uchiha clan's culture. Another might be her developing relationships with Uchiha characters in a new way, like a deeper friendship or a romantic relationship with an Uchiha.
One great Sasusaku Sakura Uchiha fanfiction is 'A New Beginning'. It focuses on Sakura's growth after the war and her relationship with Sasuke. The story beautifully portrays her as a strong and independent character, and the interactions between her and Sasuke are very heart - warming.
Some Uchiha might be suspicious of her at first. After all, she's an outsider. But others may be curious about her skills, especially if they know she's a strong ninja from her own village. They could either welcome her with open arms if they see potential benefits from her visit, or keep her at a distance if they fear she might bring trouble.
One possible plot could be that Sakura, reborn as an Uchiha, discovers new powers within herself. Since the Uchiha are known for their Sharingan, she might struggle to control this powerful dojutsu at first. She could face challenges from within the Uchiha clan as some might not accept her, seeing her as an outsider who has suddenly become one of them. Along the way, she might also have to deal with the typical Naruto -verse threats like Akatsuki, but with her new Uchiha abilities, she could offer a different perspective and set of skills to combat them.
A story where Shisui survives and he meets Sakura during a mission outside of Konoha. Sakura is initially wary of him because of his Uchiha heritage, but Shisui's gentle nature quickly wins her over. They then team up to complete the mission and in the process, discover they have a lot in common. They start to exchange medical and combat knowledge, which deepens their bond.
There might be a rival character. This could be another young ninja who is jealous of Sakura's new - found status as an Uchiha soldier. They could constantly challenge Sakura, both in terms of her skills and her place within the Uchiha - related military structure. This rivalry would add drama and tension to the story.
One idea could be a time - travel fanfic where Sakura goes back in time and meets a young Shisui Uchiha. She could be on a mission to change certain events in the Uchiha clan's history and ends up forming an unexpected bond with Shisui. They might work together to prevent some of the tragedies that befell the Uchiha.