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.
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.
Well, the plot could involve Naruto coming across a powerful being from the Dragon Ball universe who decides to teach him the Kamehameha. Naruto, being the eager learner he is, starts practicing it. He might struggle at first because the chakra control required for the Kamehameha is different from his normal jutsu. But as he trains more, he gradually gets better. He could then use this new skill in a big battle against a common enemy, surprising his friends and foes alike.
Naruto's understanding of different forms of power would expand. The Kamehameha is based on ki energy, which is different from chakra. This would make him more open - minded about other types of energy and power sources. He might start to explore if there are ways to combine chakra and ki, which could lead to him developing entirely new jutsus. This exploration could also make him more respected among the ninja community as a pioneer of new power - combining techniques.
Perhaps a powerful villain has the ability to combine different powers from different universes. This villain targets Naruto and in the process of fighting against this threat, Naruto somehow absorbs the knowledge of the Kamehameha. He then has to figure out how to use it in his own ninja - style way.
Well, one possible plot could be that they have a simple yet beautiful wedding on Kame House island. All their friends from the Z - fighter group come to celebrate. Krillin might be super nervous and 18 would be the calm one, reassuring him. And then they could start their life together, with Krillin teaching martial arts to local kids and 18 helping out in the community in her own unique way.
Another one is 'Love's Surprise'. Here, the krillin and 18 kiss happens in a rather unexpected situation. It's a short but really cute fanfiction that gives a new perspective on their relationship.