In fan fiction, their characters can be developed by giving them new goals. Android 18 could aim to become a better mother to Marron, and Krillin could strive to be a more respected martial artist in the community. They could support each other in achieving these goals. Android 18 could use her strength to help Krillin train, and Krillin could offer his wisdom to Android 18 in parenting. This would add new layers to their characters and their relationship.
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.
One interesting Super Krillin fan fiction could be about Krillin becoming the strongest Z - fighter through a new kind of training. Maybe he discovers a hidden power within himself that no one knew existed.
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.
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.