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.
Goku being born with SSJ4 in fanfic would likely make him a force to be reckoned with from infancy. This could result in quicker power-ups, different battles, and maybe even a shift in the balance of power among the characters.
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.
One possible plot could be Goku from the SSJ4 form crossing over into the world of Dragon Ball GT. He could encounter new threats that require his ultimate form to defeat. Maybe he teams up with the other Z - fighters in that universe to face off against a powerful and ancient evil that was sealed away long ago and has now been unleashed.
In Dragon Ball fanfic, Goku SSJ often has super strength, like being able to lift extremely heavy objects with ease. He can also move at incredible speeds, making it hard for enemies to even see him coming. His energy blasts are much more powerful than in his normal form, capable of destroying large structures or powerful foes in one shot.
The plot might involve Goku SSJ4 teaming up with some unexpected allies in the Tournament of Power. He could be using his SSJ4 powers in creative ways, like combining them with the techniques of his new allies to gain an edge over the other universes' competitors.
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.