In the fanfic, Sasuke's character development might involve him coming to terms with his past traumas in a new way. The police work could be a form of redemption for him. He could start to see the value of protecting others on a larger scale. Sakura's development could be centered around her growing leadership skills. As she and Sasuke solve cases, she could take on more responsibility within the team. She might also develop a tougher exterior, not being as easily intimidated, especially when dealing with dangerous criminals. Their relationship would be a key factor in their character development, as they influence each other positively.
Sasuke's character could develop by becoming more open and trusting. In the police environment, he would have to rely on his teammates, especially Sakura. He might start to let go of some of his lone - wolf tendencies. For example, he could learn to communicate better with Sakura about his plans instead of just going off on his own. Sakura, in turn, could become more confident in her abilities. Working alongside Sasuke, who is highly skilled, would push her to improve herself. She might also become more assertive when it comes to making decisions during investigations.
Sasuke in the police fanfic could develop his sense of justice in a more nuanced way. Working in the police force, he would encounter different types of criminals and situations that make him re - evaluate his views. He might become more empathetic towards victims, which is a big change from his initially more aloof self. Sakura's character could evolve in terms of her physical and mental strength. She could train more rigorously to be able to hold her own in dangerous situations. Her relationship with Sasuke would also help her in her emotional growth. She could learn to handle his mood swings better and be a more stable presence for him, while he could learn to appreciate her more for who she is.