A cool plot could be that Turles gets stranded on Earth and loses his memory. Goku finds him and takes him in. As Turles recovers, he starts to develop a new sense of self, influenced by Goku's kindness. But when his past memories start to return, he has to decide whether to go back to his old ways or stay with Goku and protect the Earth.
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.
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.
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 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.
One idea could be that Goku and Goku Black are forced to team up against an even greater threat from another universe. They start to understand each other's fighting styles during the battle and gradually develop a grudging respect for one another.
Maybe the goddess has a prophecy about Goku being the key to saving her realm from an ancient evil. So she forms a bond with Goku, and as they get to know each other better, Goku starts to develop feelings for her. Their relationship then becomes a driving force for him to fulfill the prophecy and defeat the evil, all while exploring the unique aspects of their two different worlds.
In a fanfiction, there could be a scenario where an ancient prophecy foretells of a combined power of Trunks and Goku being the only thing that can stop an impending doom. They start training together to master a special fusion technique that no one has ever attempted before. Along the way, they face challenges not only in their training but also in dealing with their own egos and differences in fighting styles.
A really interesting plot could be set in an alternate universe where Goku and Launch are arranged to be married for some political reason in the Saiyan or Earthling world. They start off not really liking each other but as they face challenges together, they fall in love. For example, they have to deal with a group that is against their union and in the process, they learn about each other's strengths and weaknesses.
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.
One idea could be that the OC is from a different dimension and has a unique power that Goku Black is intrigued by. He decides to use the OC to further his own evil plans at first, but then starts to develop unexpected feelings. Another plot could be the OC being a time traveler who tries to stop Goku Black's rampage, but gets caught in his web of lies and manipulation. A third option is the OC is a Saiyan hybrid who is initially on Goku Black's side because she is misled by his false charm, and then has to face the truth and make a difficult choice.
One idea could be exploring their relationship in an alternate universe where they succeed in their plans. Another could be a story where Goku Black has a change of heart due to his connection with Zamasu and tries to stop their destructive ways. Maybe a plot where they face a new enemy that forces them to reevaluate their alliance.