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.
The 'lemon' in an Android 18 fanfic could imply a sense of freshness. Her character is already quite unique, being an Android. So, the fanfic could play on that by having her bring a new perspective to the story. For instance, she might introduce new technology or ways of thinking to the other characters.
Another storyline could be a time - travel one. Android 21 somehow gets sent back in time and meets a younger Goku. This would be really interesting as she would have knowledge of future events and technologies. Goku, being his curious self, would be intrigued by her and want to learn from her. They could team up to prevent some of the future disasters that Android 21 knows about.
One great storyline could be them teaming up against a new, powerful enemy that neither can defeat alone. They start off not trusting each other fully as they have different fighting styles. Android 17 is more of a lone wolf type, while Trunks is used to fighting alongside his friends. But as they battle, they learn to work together, with Android 17 teaching Trunks some of his unorthodox combat moves, and Trunks showing 17 the value of friendship and teamwork in a fight.
One interesting storyline could be about Naruto having a different power origin. For example, instead of being the host of the Nine - Tails, he might gain his power from an ancient sage. This could change the whole dynamic of his character development and relationships in the story.
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.
One possible storyline could be them teaming up for a tournament. Gohan's martial arts skills combined with 18's strength would make for an interesting duo in the fighting arena. They could face various opponents and learn to trust each other's abilities more.
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.