Well, obviously Turles and Goku are the main characters. Turles is often depicted as the villain in the original story, but in fanfiction, he can be portrayed in different lights. Goku, on the other hand, is the hero. Their interactions and the development of their relationship form the core of the fanfiction.
Maybe a fanfic where Goku discovers Turles has a different side to him. Turles could be in a situation where he has to protect something or someone innocent, which makes Goku see him in a new light. They then have to work together to keep this innocent thing safe from other villains in the Dragon Ball universe.
One idea could be a story where Goku and Turles are forced to team up against a new, even greater threat to the universe. They start off distrusting each other due to their past but gradually learn to work together and respect one another's strength.
By having them share common goals. For example, if there's a powerful artifact that both want to keep out of the wrong hands. This would force them to interact and start to understand each other's motives.
First, think about the basic plot. Decide if it's going to be an adventure, a character - study, or something else. For example, if it's an adventure, figure out what kind of adventure they'll go on.
One possible plot could be Goku and Turles forced to team up against a new, powerful enemy. Maybe a villain from another dimension that threatens both their home planets. This would force them to put aside their differences and combine their unique fighting styles.
Perhaps in a different universe, Turles and Goku are brothers. They grow up together, training side by side, but then a prophecy divides them as they are told one must defeat the other for the greater good. This creates a complex relationship full of love, rivalry, and moral dilemmas.
Well, perhaps in this fanfiction, Goku Black makes a fatal mistake. He could underestimate Goku's ability to adapt during battle. Goku, being the experienced fighter he is, notices this weakness and uses it against Goku Black. He might use a combination of his signature moves like the Kamehameha in a new and unexpected way that catches Goku Black off - guard, leading to his defeat.
Their relationship could develop through mutual respect for each other's strength. Goku, being the pure - hearted Saiyan he is, might start to see that Turles has potential for good deep down. Turles, on the other hand, could be in awe of Goku's power and gradually change his view of Goku from an enemy to a rival and perhaps even a friend.
The story could start with Goku being pushed to his limits by Goku Black's attacks. But then, he remembers his training with the Kai's and how they taught him to focus his energy in a more efficient way. He then uses this knowledge to create a more powerful Kamehameha that breaks through Goku Black's defenses and kills him.