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.
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.
The main interesting part is the exploration of different relationships. In Dragon Ball Z, most of the focus is on the male characters and their battles. But in yuri fanfics, we get to see female characters in new lights, their friendships and possible romantic relationships.
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.
It probably wouldn't fit directly into the main canon as it's a rather light - hearted and comical concept. But it could exist as a side - story or an 'alternate universe' type of fanfic. It can show the characters in a more relaxed and fun - loving situation that we don't often see in the main series.
The Saiyan OC could be a time - traveler who comes to the Dragon Ball Z era. Android 18 meets this strange Saiyan and at first is suspicious. But as they get to know each other, they realize they can work together. They might have to deal with the chaos that the Saiyan OC's time - traveling has caused in the timeline. Maybe some old villains are resurrected because of it, and Android 18 and the Saiyan OC must find a way to stop them while also dealing with their own relationship development.
A possible plot could involve an enemy who discovers Android 18's ticklish feet and tries to use it as a distraction during a fight. However, Android 18, being the strong - willed character she is, manages to overcome this odd form of attack and turn the tables on the enemy. This would add a unique and humorous twist to the typical Dragon Ball battles.
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.
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.
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.
One interesting storyline could be Android 21's struggle to find her true identity. In fanfiction, she might discover that she has a hidden past or purpose that was not initially known in the original series.