Well, it could be about Goku Jr learning some of Goku's most powerful moves. Maybe Goku teaches him the Kamehameha. They might also go on an adventure together, like fighting a new and powerful enemy that neither could face alone.
In this fanfiction, they might have an epic sparring session. Goku could pass on some of his old fighting techniques to Goku Jr.
Obviously, Goku and Goku Jr are the main characters.
Maybe an old enemy of Goku's who has somehow managed to return. This enemy has heard about Goku Jr and figures that by attacking him, he can get revenge on Goku. This could be someone like Frieza who has found a new way to be resurrected and is more powerful than ever.
In a 'Bardock meets Goku fanfiction', perhaps Bardock is pulled through time and space and lands on Earth during one of Goku's battles. Goku is shocked to see his father but quickly accepts him. Together, they team up against a powerful villain. Bardock shares his Saiyan knowledge with Goku, which helps Goku unlock new powers and defeat the enemy.
One possible plot could be that Goku, through some time - travel or interdimensional rift, meets Bardock in the past. Bardock could initially be suspicious of Goku, not knowing he's his son. But as they interact, Goku might show some of his Saiyan power and spirit that reminds Bardock of his own. They could then team up against a common enemy, like a powerful alien race threatening Planet Vegeta.
There could also be a moment of self - discovery for both of them. Goku might see his past self in Kid Goku and realize how far he has come, while Kid Goku could get a glimpse of his future potential through meeting his adult self. They could share stories about their friends and enemies, like how Goku could talk about Vegeta and Frieza to Kid Goku, and Kid Goku could tell Goku about his early battles with the Red Ribbon Army.
Perhaps they would team up against a common enemy. In fanfiction, new threats are often created. They could combine their strength and unique techniques to defeat the bad guy.
Another potential rival could be a young, prodigious martial artist from a new school of martial arts on Earth. This character could be jealous of Goku Jr's fame and constantly try to outdo him in battles and in the eyes of the public. He might use unorthodox techniques that Goku Jr has to quickly figure out how to counter.
Well, it could be that Goku meets his mother in a dream world. Maybe she gives him some advice on how to become even stronger. She might share some long - forgotten family secrets with him too.
One possible plot could be that Bardock and Gine somehow travel through time or dimensions to meet Goku on Earth. They could be shocked to see how strong and kind - hearted Goku has become, despite not growing up on Planet Vegeta. Goku, in turn, would be amazed to meet his parents and learn more about his Saiyan heritage from them.