Well, this fanfic could be quite interesting from a character - development perspective. If Cell kills Gohan, it could change the entire dynamic of the Dragon Ball world. For example, Vegeta might be forced to step up in a different way, or the Z - fighters might have to completely reevaluate their strategies. It might also show a different side of Cell, perhaps a more menacing and victorious side that we haven't fully seen in the original series.
One possible theme could be the fragility of life. Even a powerful fighter like Gohan can be killed, showing that no one is invincible. Another theme might be the concept of revenge. If Gohan is killed by Cell, other characters could seek revenge against Cell.
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.
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.
Love and affection could be a major theme. The 18 kisses are a clear sign of that. Another theme might be acceptance, as Krillin accepts 18 for who she is, an android, and she accepts him with all his flaws.