Perhaps the fanfic could be a time - travel story. Android 21 accidentally sends Goku back in time to a point where she was first created. Goku then tries to prevent her from becoming the villain she was meant to be. This would involve Goku having to deal with the scientists who created her and convincing them to take a different path in their research, all while trying to find his way back to his own time.
One possible plot could be that Android 21, with her unique powers, teams up with Goku to face a new and powerful enemy. They might have to combine their strength and fighting techniques to defeat this foe.
In a fanfic, it could be that Android 21 has a hidden power that only Goku can unlock. As Goku helps her discover this power, they form a deep bond. This power could be related to the Dragon Balls in some way, like having the ability to control the Dragon Balls' wishes or even create new Dragon Balls.
It could also be a fanfic centered around their friendship. Android 18 might initially be cold towards Bulma, but as time goes on, they start to bond. Bulma could introduce Android 18 to the joys of normal life, like shopping or going to the beach. And Android 18 could protect Bulma from threats that her scientific work sometimes attracts. In the end, they become inseparable friends, changing the way the other characters view 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.
Well, one possible fanfic could be about Android 18 and Raditz teaming up against a common enemy. Maybe they initially start off as reluctant allies, but as they fight side by side, they begin to respect each other's strength.
Well, in a fanfic, Kid Goku and Android 18 could have an interesting dynamic. Kid Goku is pure - hearted and always eager for a good fight. Android 18, on the other hand, is tough and has her own sense of justice. Their interaction might start with a misunderstanding. Maybe Goku mistakes 18 for a bad guy at first because she's an android. But then, as they get to know each other, they could end up teaming up against a common enemy. Goku would be impressed by 18's strength, and 18 might be a bit amused by Goku's naivete.
Another way is that Goku could use the Spirit Bomb. He could gather energy from all living things on Earth and nearby planets. Android 21, being an artificial being, might not be able to withstand the pure energy of the Spirit Bomb. However, she could try to absorb the energy, but if Goku can overload her absorption capabilities, it would be his victory.
They could go on a quest to find a powerful dragon ball that has been lost. Goku's sense of adventure and 18's combat skills would be a great combination. Along the way, they would face various enemies. Goku would use his super - fast speed and powerful attacks, while 18 would use her android abilities like energy absorption to deal with the threats.
It could start as a mutual respect for each other's power. Goku would admire Android 21's combat abilities, and Android 21 would be intrigued by Goku's Saiyan strength.
One common theme is empowerment. Since Goku is already a very powerful character, in these fanfics where she's female, it often emphasizes her strength even more, showing that gender doesn't limit her abilities.