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.
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.
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.
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 storyline could be Goku and Android 18 teaming up to fight a new, powerful enemy that neither of them can defeat alone. They have to learn to combine their skills and powers in unique ways.
A Dragon Ball Android 21 fanfic is interesting when it adds something new to the existing lore. Maybe it gives a new origin story for Android 21 that ties in with the overall Dragon Ball mythology in a creative way. It could also explore her place in the universe from a different perspective, like how she might view the Saiyans' obsession with power or how she could use her scientific knowledge in ways that the original story didn't. Additionally, a good fanfic will have well - written dialogue that really captures the essence of the characters, especially Android 21's internal conflict and her external interactions.
A third possible plot could be a story where Goku and Bulma are separated from the rest of the Dragon Ball gang. They have to rely solely on each other to survive and find their way back. During this time, they discover new powers within themselves and also learn more about each other's pasts, which brings them closer together.
One interesting plot could be a time - travel scenario where Goku and Bulma end up in an alternate timeline together. They might have to face new threats that are different from the ones in the main story. Another could be a story where they go on an adventure to a new planet and discover a powerful energy source.