You can try Archive of Our Own (AO3). It has a large collection of fanfictions from various fandoms, and there's a good chance you'll find some great kazuma x yunyun fanfiction there. Just use the search function and type in the relevant tags.
One great kazuma x yunyun fanfiction could be 'The Adventure Together'. It might focus on their journey through a new and dangerous dungeon, where they have to rely on each other's skills. Kazuma's strategic thinking combined with Yunyun's magic could make for an exciting plot.
There could be a fanfiction where Kazuma and Darkness get transported to a different realm. In this realm, they have to adapt to new rules and powers. Their relationship deepens as they struggle to find a way back home, and along the way, they discover new things about themselves and each other.
Some fanfictions may focus on a 'what if' scenario, like what if Kazuma and Aqua were transported to a modern - day Earth? They would have to adapt to new technologies and cultures, which would lead to humorous and heartwarming situations as they rely on each other more than ever.
Growth is also a common theme. Both hiruma and sena grow as individuals, and their relationship changes as they do. For example, sena may become more confident because of hiruma's influence, and hiruma may become more understanding. Also, the theme of loyalty is prevalent. They are loyal to each other in the face of difficulties and opposition from others in many fanfics.
Websites like Archive of Our Own (AO3) are great places to start. It has a large collection of fanfictions from various fandoms, including ones for Kazuma and Darkness. You can search for specific tags related to them to find the stories you like.
You can start by looking on popular fanfiction websites such as Archive of Our Own or FanFiction.net. Use the search function and type in 'kazuma x aqua' to see a list of relevant stories.
One idea could be that the OC is a new transfer student in Kazuma's school. They initially clash due to different personalities, but as they get paired for a project, they start to understand each other better. Kazuma might be initially cold towards the OC, but as time passes, he realizes the OC has a unique charm.
Create shared experiences. It could be something as simple as getting caught in the rain together or something more elaborate like solving a mystery. Through these shared experiences, they can bond and grow closer. Also, show how they support each other during difficult times. If Kazuma is having a hard time with schoolwork, the OC could offer to help, and vice versa. This mutual support is crucial for relationship development.
One popular 'hiruma x sena fanfic' is 'The Bond Beyond the Field'. It beautifully portrays their relationship growth from teammates to something more. Another is 'Under the Stars', which focuses on their quieter moments together, showing a deeper emotional connection.