Their relationship could grow deeper as they face the challenges of pregnancy together. Krillin might become more in tune with 18's needs, and 18 could see a new side of Krillin's caring nature.
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 possible fanfiction could focus on their daily life together. They might have a sweet morning routine, like making breakfast and chatting about their plans for the day. Krillin could be shown as a very caring partner, always looking out for Android 18's well - being. For example, he might bring her a flower he found during his training, just to make her smile.
There are likely some fanfics about Krillin and Android 18 that focus on their relationship in a non - sexual and family - friendly way. You can try looking on fanfiction websites like FanFiction.net or Archive of Our Own. Search for stories tagged with 'Krillin' and 'Android 18' to find ones that might interest you.