Obviously, Sakura and Sasuke are the main characters. Even though Sakura forgets Sasuke, their relationship (or the lack of it in this case) forms the core of the story.
One idea could be that Sakura forgets Sasuke due to a magical spell gone wrong. This could lead to her living a completely different life, free from her past obsession with him.
One possible plot could be that Naruto suffers a head injury during a battle. As a result, he forgets about Sasuke completely. His friends initially don't notice, but as time passes, they realize Naruto's behavior towards Sasuke - related things is strange. For example, he doesn't get angry when Sasuke is mentioned like he used to. Then they try to help Naruto regain his memories of Sasuke through various means, like showing him old photos or taking him to places they used to go together.
It could be a rogue ninja who has a vendetta against Sasuke. This ninja knows that Naruto's bond with Sasuke is strong, so he uses a special jutsu to make Naruto forget Sasuke, hoping to break Sasuke's spirit. By removing Naruto from Sasuke's life in this way, the rogue ninja believes he can defeat Sasuke more easily.
Since 'hard lemon' fanfiction is inappropriate, we can talk about their general characteristics in normal fanfiction. Sasuke is often depicted as a brooding and powerful character, while Sakura is shown as strong - willed and loving. In non - inappropriate fanfiction, their relationship might be shown as evolving from initial acquaintance to deep love, with Sasuke's cold exterior gradually melting due to Sakura's unwavering affection.
Naruto is often a popular character in these fanfictions. He could be Sasuke's rival for Sakura's attention or a good friend who tries to help them get together. Ino is also a common character. She might be Sakura's friend or sometimes a rival in matters related to Sasuke. Kakashi could be their teacher who gives them advice about their relationships and schoolwork.
In the hospital setting, Sasuke might initially be his aloof self. But as Sakura takes care of him or they face challenges together, he could start to open up more. For example, if Sakura is overworked, Sasuke might offer a helping hand in small ways, like getting her coffee. This would show his growing affection for her.
Their relationship can be shown as one filled with deep love and care. Sasuke, who has always been a bit aloof, becomes much more tender and attentive towards Sakura during her pregnancy. He might start doing small things like getting her favorite foods or just sitting with her and talking about their future as a family.
Sakura might develop into a more confident and assertive person. After the war, she has proven herself as a strong kunoichi, and with Sasuke by her side, she could grow even more. Sasuke, on the other hand, would develop by learning to trust and love again. He has been through so much, but with Sakura's love and support, he could become a more stable and caring person.
The relationship can also be developed through their interactions with other characters. For instance, if they have a child, how they both interact with the child and how their parenting styles differ and blend. Also, how they interact with their friends like Naruto and Hinata. Maybe they have double dates or group outings. All these interactions can show different aspects of their relationship and how it grows and evolves over time.