The development can also be based on personal growth. Horikita is often very focused on her own goals and can be a bit cold. Ayanokoji could be the one to help her see the importance of relationships. Maybe he shows her how a more empathetic approach can help in achieving their class's success. This would lead to Horikita opening up to him more, and Ayanokoji in turn would start to show more of his true self to her. Over time, a deeper relationship could form as they both grow as individuals within the context of the fanfic.
One plot idea could be that Ayanokoji and Horikita are paired up for a special project in the school. As they work together, Horikita starts to notice the hidden depths of Ayanokoji's character that she hadn't seen before. This makes her view him in a new light and their relationship begins to develop.