One plot could be that Android 21 has a family in her past that she doesn't remember. Goku helps her to uncover her past and find her family. Along the way, they face many challenges and enemies who don't want Android 21 to discover her true origins. This would be a more emotional and character - driven plot in the Dragon Ball Z universe.
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.
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 interesting plot could be Dark Goku's origin story. Maybe he's not just an evil version of Goku but has a more complex backstory involving some dark magic or a different dimension's influence. Another plot might involve his battles against the Z - fighters. How he uses his unique powers against them and the strategies they come up with to counter him.
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 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.
One interesting plot could be Goku traveling back in time to train with his younger self. He could teach his younger version some new techniques, like the Instant Transmission at an earlier stage. This would change the power dynamics in the earlier story arcs and lead to different battles against foes like Vegeta. Maybe the Z - Fighters would be stronger and better prepared for future threats.
One interesting plot could be Goku's daughter having to train in a new dimension. She discovers unique powers there and has to use them to save the Earth from a new threat. This would be a great way to explore new aspects of the Dragon Ball Z universe while also focusing on a new character, Goku's daughter.
One interesting plot could be Goku and Bulma navigating the challenges of combining their very different lifestyles. Goku, being a Saiyan warrior, is used to constant training and battles, while Bulma is more into technology and inventing. In the fanfiction, they might struggle to find common ground in raising their family. For example, Goku might want to train their children in martial arts from a very young age, but Bulma could be more concerned about their academic education and safety. Another plot could be them facing threats from enemies who target their relationship. Maybe an old foe of Goku sees his marriage to Bulma as a weakness and tries to use it against him.
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 interesting plot could be Evil Goku's origin. Maybe he's a result of some dark magic or an alternate dimension's influence. Another plot might involve his battles against the regular Z - fighters. He could have new and more devious fighting techniques.