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.
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.
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 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.
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.
You can try websites like Archive of Our Own (AO3). It has a large collection of fanfictions, including Chishiya x Reader ones. Just use the search bar and type in 'Chishiya x Reader' and you'll get a bunch of results.
Some Niragi fanfics could be about his relationships with other characters. Maybe a fanfic where he forms an unlikely alliance with someone he initially hated. This could be really interesting as it would show a different side of Niragi, a side that might be more complex than his one - dimensional 'villain' persona in the original story.