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.
Their relationship could develop into a deep friendship. Goku, being the kind - hearted fighter he is, would likely accept Android 18 for who she is. And Android 18, seeing Goku's strength and good nature, would start to respect him more and more.
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.
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.
Well, they might become friends. Goku, being the kind - hearted boy he is, would probably be curious about Android 18. Maybe they'd start training together, with Goku showing Android 18 some of his martial arts moves. And Android 18 could show Goku some of her advanced technology - like knowledge, if she had any in her android form even as a kid. They could also get into all sorts of adventures, exploring the world around them and getting into harmless little troubles.
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.
I can't really provide you with a particular 'goku and android 18 lemon fanfiction' as there are so many out there with varying quality. You could also try reaching out to other Dragon Ball fans on social media platforms. Ask them if they know any good fanfictions in this category. This way, you might get some more personalized recommendations based on others' experiences.