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.
A possible relationship could be a friendly rivalry. They are both powerful in their own ways. Goku is a Saiyan with incredible physical strength and combat abilities, while Android 21 has her own set of unique powers. They might constantly challenge each other to friendly spars, which would help them both improve their skills and also deepen their understanding of each other.
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 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.
Well, it could entail their adventures together. Maybe it focuses on how they first met and started their journey to find the Dragon Balls. Their relationship is quite interesting as they are from different backgrounds, with Goku being a naive but powerful Saiyan and Bulma being a smart and resourceful Earthling.
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.
Goku and Bulma have many exciting adventures. They first set out to find the Dragon Balls together. Along the way, they encounter various foes and challenges. Bulma provides the technological know - how, like using her Dragon Radar to locate the balls, while Goku uses his martial arts skills to protect them from danger.
They could go on a quest to find a special ingredient for a meal that Chi - Chi wants to cook. Goku, being Goku, might make the journey more exciting with his superhuman abilities and his knack for getting into trouble.
A possible plot could be that Android 18 and Goku are thrown into an alternate dimension where they have to rely on each other to survive. In this dimension, they face challenges that test their friendship and fighting skills. Android 18 might learn some of Goku's signature moves and Goku could learn more about the technology within Android 18.