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.
I'm not really sure if they end up together. Different people might have different interpretations. Maybe you can dig deeper into the manga and form your own conclusion.
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.
Themes like friendship and rivalry are quite common. In 'naruto x cote fanfiction', Naruto might form new friendships with the COTE characters, or he could become rivals with some of them due to their different goals and personalities. Also, the theme of self - improvement is there. Naruto's journey in the COTE world could be about learning new things and growing as a character in a different context.
In the light novel, Ayanokoji is clearly the more intelligent one. He has a unique ability to analyze situations and people's motives deeply. Ryuen often falls into Ayanokoji's traps because he doesn't fully understand Ayanokoji's true nature. Ayanokoji can anticipate Ryuen's moves and counter them effectively, while Ryuen struggles to keep up with Ayanokoji's complex thinking.