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.
You can try websites like Archive of Our Own (AO3). It has a large collection of fanfics, and you can easily search for kiyotaka x makoto fanfic there. Fanfiction.net is also a good place. Just use the search bar and type in their names to find relevant fanfics.
One great kiyotaka x makoto fanfic is 'The Unlikely Bond'. It beautifully explores their relationship development. Another is 'Together Against All Odds' which has an interesting plot that shows how they support each other through various challenges. And 'A New Friendship' is also a good one that focuses on the early stages of their relationship.
One common theme is friendship development. Writers often explore how their relationship changes from being just classmates or acquaintances to something deeper, whether it's a strong platonic bond or something more romantic.
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.