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.
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.
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.
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.
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.
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.
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.
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.
The main characters often include Dark Goku and the Z - fighters. Among the Z - fighters, Piccolo is an important one. His wisdom and combat abilities can be crucial in dealing with Dark Goku. Krillin also has his place. His long - standing friendship with Goku and his own fighting skills add depth to the story. And Master Roshi might also make an appearance, perhaps as an advisor or someone who has some knowledge about an ancient power that could be relevant to the fight against Dark Goku.
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.