There is a fanfic called 'Beyond the Battlefield'. In this story, Goku and Android 18 are forced to team up for a mission. As they work together, they start to understand each other better. The author does a great job of writing their dialogues, which are full of humor and some underlying tension.
A possible plot could be that Android 18 and Goku are thrown into an alternate dimension where they have to rely on each other to survive. In this dimension, they face challenges that test their friendship and fighting skills. Android 18 might learn some of Goku's signature moves and Goku could learn more about the technology within Android 18.
In a fanfic, it could be that Android 21 has a hidden power that only Goku can unlock. As Goku helps her discover this power, they form a deep bond. This power could be related to the Dragon Balls in some way, like having the ability to control the Dragon Balls' wishes or even create new Dragon Balls.
In some fanfics, other characters might also play a role. For example, Krillin, who was once in a relationship with Android 18. His presence could add an interesting dynamic, like jealousy or a sense of competition, to the relationship between Goku and Android 18.
Character development is key. In a good 'Goku x Android 18 fanfic', we should see both characters grow and change because of their relationship. For example, Goku might become more in - tune with his emotions, and Android 18 could become more open. Also, an interesting plot that goes beyond the typical fight - and - save - the - world scenario is great. Maybe something like them exploring a new world together or having to solve a mystery.