They might start with some hesitant glances and then progress to small touches during their training sessions together. Eventually, they could share some passionate embraces.
Well, it could be a relationship based on power dynamics. Goku is extremely powerful, and Android 21 is no slouch either. They might engage in battles that gradually turn into a form of respect for each other's abilities. And then, this respect could evolve into a deeper connection. For example, Goku could teach Android 21 about the joys of a fair fight, and she could in turn show Goku some of her unique Android - related skills or knowledge.
Maybe in the fanfiction, Goku accidentally uses a new technique that causes him and Android 18 to be mentally linked. Through this link, they start to understand each other on a deeper level. They then find themselves drawn to each other's strength and determination. But they have to hide their growing relationship from their friends and family who might not understand.
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.
A possible plot could be that Android 18 gets injured during a fight and Goku takes it upon himself to protect and heal her. This close proximity leads to the development of feelings, which is where the 'lemon' part might come in. Their relationship could be a secret at first as they are from different 'worlds' in a sense, with Goku being a Saiyan hero and Android 18 being an android once created for destruction.
The interaction might start with suspicion. Goku, always on guard for threats to the Earth, would be cautious around Android 21 at first. But Android 21 could be intrigued by Goku's power and simplicity. As time goes on, they could start training together, which would be an interesting interaction as Goku's training methods are very different from what Android 21 is used to.
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.
It might also be a story about self - discovery. Android 18 could be exploring her more human side with Goku's influence. Goku, on the other hand, could learn more about the androids and how they are not all just mindless killing machines. The 'lemon' aspect might imply some sort of budding relationship that is full of passion and challenges as they come from very different backgrounds.
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.
Chi - Chi might be one of the first to support them. Despite her past relationship with Goku, she could see how happy Goku is with Android 18 and decides to be happy for them. Also, Krillin would likely support the marriage. He has a deep friendship with both Goku and Android 18. He knows Android 18 well and trusts Goku, so he would be all for their union.