Well, in fanfiction about Saiki Kusuo and Teruhashi, often their relationship is explored in really interesting ways. Sometimes it shows Teruhashi's attempts to get closer to Saiki in a more comical way compared to the original series. And Saiki, with his unique powers, might react in different and unexpected manners.
In the manga, Saiki doesn't have a romantic ending with Teruhashi. The focus is more on Saiki's supernatural powers and the humorous situations he gets into rather than a traditional romance.
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 popular theme is the 'unlikely couple' aspect. Given Saiki's introverted nature and Teruhashi's popularity, the idea of them getting together is an interesting concept. Another theme is the comical misunderstandings. Since Saiki has psychic powers and Teruhashi is always trying to be perfect, there are a lot of opportunities for misunderstandings that lead to humorous situations.
There's 'Saiki and Teruhashi: Unlikely Bond'. In this fanfic, the author delves into the idea of Saiki slowly realizing that Teruhashi is more than just the perfect - looking girl everyone admires. It shows how their relationship evolves from Saiki being constantly annoyed by her to actually starting to understand her on a deeper level. It has some really sweet moments that fans of the pairing will love.