You can try fanfiction websites like Archive of Our Own (AO3). It has a large collection of various fanfics, including 'hikigaya x harem' ones. Just use the search bar and type in relevant keywords to find what you're looking for.
One popular 'hikigaya x harem fanfic' could be 'The Hikigaya's Harem Adventures'. It often features interesting character interactions and unique plotlines that revolve around Hikigaya and his relationships within the harem. Another one might be 'Hikigaya in the Harem World' which delves into how he copes with multiple characters vying for his attention in a fictional world.
One great 'hikigaya x yukinoshita fanfic' is 'A Different Perspective'. It delves deep into their relationship, showing how their personalities interact in new and interesting ways outside of the original story's context.
One common theme could be friendship development. They might start as acquaintances and gradually become closer friends, sharing secrets and having deep conversations.
One common theme is the discovery of self - worth. Both characters often have insecurities, and through their relationship in the fanfic, they start to see their own value more clearly.
The writing style matters a great deal. A fanfic that uses vivid language to describe their emotions and interactions is more engaging. For instance, when describing the moments of their growing closeness, detailed descriptions of their body language, the looks in their eyes, and the words they exchange can make the story really come alive.
Growth is also a major theme. They grow both individually and as a pair. For example, Hikigaya might become more open - minded and Kaori might become more understanding. Love is, of course, a prevalent theme too. The development of romantic feelings between them is often explored in these fanfics.
I'm not sure about specific saki x hikigaya fanfic titles off the top of my head. But often, if you join relevant fan communities on social media platforms or forums dedicated to the source material (assuming there is a particular anime or manga they are from), fellow fans can recommend some great ones. They might even share their own fanfic creations in those communities.
There's a fanfic titled 'Shared Moments'. In this one, it delves deep into the emotional connection between hikigaya and kawasaki. It shows how they gradually understand each other better over time through various shared experiences at school and in their daily lives. It has really well - written dialogue that brings their relationship to life.
Some forums dedicated to the series might also have links to good fanfics. For example, the official forums for the anime or light novel might have threads where fans share their favorite 'hikigaya x yukinoshita fanfic'.