One good theme could be their family life. How they raise Marron and deal with the daily challenges of being parents in the Dragon Ball world.
Maybe a story where 18 and Krillin decide to open a martial arts school. They have to deal with different types of students, and it shows their relationship growing as they work towards a common goal. It can include how Krillin uses his knowledge and 18 her strength to train the students.
Another place to check could be dedicated Dragon Ball fan forums. There are often threads where fans share their own fanfics or recommend ones they've read. Some of these forums might have a specific section for family - centered fanfics which could be where you'd find Android 18, Krillin and Marron stories. You might also try social media groups dedicated to Dragon Ball. Fans sometimes post links to their fanfics there.
The fanfiction could explore their relationship growth over time. How they gradually became closer and more in - sync with each other. It could show how they supported each other through tough times, like when Krillin was worried about protecting the Earth from new threats and Android 18 was there to give him confidence and strength.