It could be a story where Naruto's positive energy starts to influence Android 21's darker side. Android 21 has a somewhat sinister side to her in the original context. But Naruto's infectious optimism and his belief in the good of people could start to change her. They might go on a journey to find a way to completely purge Android 21 of her negative tendencies, and in the process, they develop a deep bond.
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.
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.
Well, it could start with Naruto on a mission in a strange new land that turns out to be a place where Android 21 exists. They meet and initially misunderstand each other's motives. However, as time passes, Naruto discovers Android 21's softer side. Maybe she has a hidden past that Naruto helps her come to terms with. And in the process, they fall in love. Their relationship could also face opposition from both Naruto's ninja comrades who are suspicious of Android 21, and Android 21's own creators or associates who don't want her involved with Naruto.
A possible plot could be that female Android 17 is captured by Orochimaru. Orochimaru wants to study her technology to enhance his own powers. Naruto, upon hearing this, decides to rescue her. Along the way, he has to face Orochimaru's minions and various traps. When he finally reaches Android 17, she is initially distrustful but Naruto convinces her that they need to work together to escape. They then form an unlikely alliance and fight their way out of Orochimaru's lair.
One possible plot could be that Android 21 somehow ends up in the Naruto universe. Maybe she is initially seen as a threat due to her power. Naruto, being the kind - hearted hero, tries to understand her instead of attacking right away. They could team up to fight a common enemy that has the potential to destroy both their worlds.
One possible plot could be a crossover battle. Naruto, with his powerful ninja skills like Rasengan and Shadow Clone Jutsu, could face off against Majin Android 21's unique abilities. Maybe they start off as enemies but then realize they have a common foe. For example, an evil entity that threatens both their worlds. Naruto could use his determination and leadership skills, while Majin Android 21 could bring her combat and energy - based powers to the table. In the end, they might combine their forces and develop a new friendship or alliance.