Romance is a very common theme. Given Saiki's unique personality and powers, it's interesting to see how female characters might fall for him or how he might navigate romantic feelings. Another theme is friendship. As mentioned before, the idea of a female friend for Saiki and their adventures together is quite popular.
One challenge is staying true to Saiki's character. He's very stoic and doesn't like to get involved in unnecessary things. So when writing with a female character, it's hard to make their interactions seem natural without forcing Saiki to be too out of character. Another challenge is creating a unique female character. There are so many existing characters in the Saiki K universe that it can be difficult to come up with a new one that stands out.
Nendo could be a great addition. His simple - minded yet loyal nature can add some humor and unexpected twists. For example, he might accidentally almost expose Saiki's powers while trying to be helpful in a situation involving Teruhashi.
One popular type of female Saiki Kusuo fanfiction is those that focus on his interactions with female classmates in a more comical way. For example, stories where a female character tries to befriend Saiki despite his attempts to keep his psychic powers a secret. Another kind might be a romantic fanfiction where a female character has a crush on Saiki and the story follows her attempts to get closer to him, like she might accidentally discover his powers and that leads to a unique bond between them.