Common themes often involve the unexpectedness of their relationship. Since Goku is a pure - hearted warrior and Android 18 was initially an enemy. In lemon fanfictions, there might be a focus on the passion that arises from this unusual pairing. Also, themes related to redemption and acceptance can be found, as Android 18 might find acceptance in Goku's world, and Goku might see her in a new light. This all gets intertwined with the more 'lemon' aspects which are often used to heighten the intensity of their relationship.
Another possible storyline is that Android 21, who is usually seen as a bit of an outcast, finds acceptance in Goku's group. Goku encourages the others to give her a chance. They then go on adventures together, with Android 21 constantly evolving not just in power but also in her understanding of the world around her.
Maybe in a post - battle scenario, Goku and Android 18 are left alone to recover. They begin to have conversations about their different origins. Goku shares about his Saiyan heritage and training, while Android 18 talks about her creation by Dr. Gero. This exchange could lead to a deeper emotional connection between them.
If Goku married a Saiyan in a fanfiction, it might bring new challenges and adventures. Maybe there'd be family feuds or combined efforts to protect their kind. It really depends on how the author envisions their relationship and the subsequent story arcs.
Often, it could be new villains created specifically for the fanfiction. Maybe a powerful android created by the same scientist who made Android 21 but with a more evil intent.
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.
In a fanfiction scenario, Goku and Android 21's fight could attract the attention of other Z - fighters. Vegeta, for example, might be reluctant to help Goku at first but then realizes the danger Android 21 poses to the entire planet. Goku could also try to reach out to Android 21's good side, as he has a history of trying to redeem his enemies. This could add an interesting moral dilemma to the story, whether to fight or try to save her.