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.
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.
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.
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.
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.
Well, a plot could be Android 21 somehow getting her hands on the Namekian Dragon Balls. Since she has that scientific mind, she might try to figure out how to modify the wishes. In the process, she could draw the attention of Frieza who wants the Dragon Balls for his own evil purposes. Then the Z - fighters would have to step in to protect Android 21 and the Dragon Balls. Another plot idea is that Android 21 is from an alternate universe where she was created by the good guys. When she crosses over to the main Dragon Ball Z universe, she has to adjust to the different power levels and politics among the characters. And she could end up being a key player in the fight against Buu, perhaps using her unique abilities to weaken Buu in some way.
In this fanfiction scenario, Goku meeting Gine could be really interesting. Gine is Goku's mother, and in a fan - made story, their meeting might involve Gine finally seeing how strong her son has become. Maybe she'd be proud of all his achievements in protecting the Earth and the universe. It could also be a very emotional encounter as Goku has grown up without her presence for most of his life.
A fanfiction might explore the idea of Bardock seeing Goku's life through a time - space rift. He witnesses all of Goku's battles and the friends he makes. Bardock is filled with pride but also realizes that Goku has a different path than the traditional Saiyan way. This makes him question the Saiyan culture he once knew. Meanwhile, Goku starts to have strange dreams about his father, which leads him to search for more about his Saiyan heritage.
One idea could be Goku Jr facing a new, powerful enemy that has a unique power related to time manipulation. This would force him to train in ways he never has before and seek help from unexpected allies, like some of the older, wiser characters from the original Dragon Ball Z series.
One idea could be Goku and Gohan going on a special training journey to a new planet with unique gravity and energy sources. This would force them to adapt and develop new techniques together.