One common theme is Saiki's struggle with his powers. In many fanfics, he tries to hide his psychic abilities from others while still having to use them at times, which leads to comical and tense situations.
One interesting saiki fanfic could be about Saiki using his powers in a completely new and unexpected situation, like saving a whole city from a strange alien invasion that only he can detect with his psychic abilities.
The contrast between their characters. Saiki with his extraordinary powers and Satou who tries to be as ordinary as possible. This contrast creates a lot of comical and heart - warming situations in the fanfic.
One common theme is the discovery of self. The OC often comes into Saiki's world and through their relationship, both characters learn more about themselves. Another theme is friendship. Despite Saiki's initial reluctance, a friendship forms which is central to many stories.
A good Kokomi x Saiki fanfic often has well - developed characters. It should stay true to the original personalities of Kokomi and Saiki while also exploring new aspects of them in the context of their relationship. For example, if Kokomi is usually calm and collected in the original work, the fanfic should maintain that essence while showing how she might act differently around Saiki.
Another theme is the idea of Teruhashi trying to break through Saiki's emotional barriers. Since Saiki is often aloof, Teruhashi's attempts to get closer to him form a major part of many fanfics. It can be really entertaining to read how she tries different strategies to get his attention.
One common theme is the reader having some sort of special relationship with Saiki. It could be a friendship or a romantic relationship. For example, the reader might be the only one who truly understands Saiki's powers and they form a deep bond.
You can try fanfiction websites such as Archive of Our Own (AO3). It has a large collection of saiki fanfics. There are different genres and styles to choose from.
One idea could be a fanfic where Saiki Kusuo has to deal with an alien invasion while trying to keep his psychic powers a secret from the new threats. Another could be a story about him going on a school trip and using his powers in hilarious ways to make the trip more interesting or avoid disasters. Maybe a fanfic where he befriends a new student who has a very unique power that even surprises Saiki.