Bulma could also be a common character paired with Krillin. Bulma is very intelligent and could potentially help Krillin figure out how to time travel better or she might be the one who creates the time - travel device in the first place. And she could use her resources to help Krillin in his time - traveling adventures.
In the manga, Krillin doesn't stop training. His sense of duty and the need to protect his comrades push him to keep improving his skills and strength.
One common theme is self - improvement. Krillin is often depicted as constantly striving to get stronger, whether it's through new training methods or unlocking hidden powers.
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.
Yamcha could potentially betray Bulma. Even though they have a history together, in a fanfic scenario, he could be jealous of Bulma's growing closeness with Vegeta or her scientific achievements. So, he might do something to undermine her, like spreading false rumors about her work.