One characteristic is that they often focus on Chishiya's intelligence. Writers like to show how he outsmarts others in various situations. For example, in many fanfics, he's the one who figures out the rules of the deadly games first.
In the Alice in Borderland manga, unfortunately, Chishiya met his end. The circumstances of his death were quite dramatic and tied to the overall plot progression.
The theme of survival in the Borderland also plays a big part. They are both trying to survive in this dangerous world, and their relationship can either be a hindrance or a help in that. In some fanfictions, they have to rely on each other despite their differences to make it through various deadly games.
Well, 'Chishiya and the Lost Memories' is quite popular. In this fanfic, Chishiya wakes up with partial amnesia and has to figure out his past while still being his intelligent self. It has a really engaging plot. Also, 'Chishiya's Allies' is good as it focuses on his relationships with other characters in a more detailed way.
Romance is also a big theme. Writers often explore the relationship between the reader and Chishiya, from the initial attraction to a deeper emotional connection. Some fanfictions focus on how they fall in love in the midst of the chaos of the Borderland.
One good 'chishiya x niragi fanfiction' could be 'The Unlikely Bond'. It explores the complex relationship between the two characters in an interesting way. They start off as antagonists but gradually develop a unique connection through various challenges in the Borderland.
One great source could be Archive of Our Own (AO3). There are numerous fanfics about Chishiya x Arisu there. You can filter by rating, length, and genre to find the ones that suit your taste.