Goku Jr. could be developed further by delving into his heritage more. He could start to have visions or dreams about the past Saiyan battles and learn from them. This could lead to him developing new power - ups based on the ancient Saiyan techniques. Additionally, his character could be developed through his interactions with the new generation of characters in the Dragon Ball GT universe. He could learn from their different skills and attitudes, and in turn, teach them about the values that Goku and the other Z - Fighters held dear.
To develop Goku Jr's character further, he could be given more complex emotions. For example, showing his fear in the face of truly overwhelming enemies, rather than always being fearless like Goku. This would make him more relatable. Also, exploring his relationships in more depth, like his friendship with Pan. Maybe they have a falling out over different views on how to protect their world, which forces Goku Jr. to grow and understand different perspectives.
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.
A plot idea could be that Goku Jr. discovers a mysterious artifact that has a connection to the Dragon Balls. As he tries to figure out its purpose, he gets caught up in a race against an evil group who also wants the artifact for their own malicious plans. This could lead to him making new allies and having to use his wits as well as his strength. He might also have to face some personal challenges, like doubting his own worthiness to be a hero, similar to how Goku in the original series sometimes had self - doubts.
Goku's understanding of different universes and races could be deepened in fanfiction. Since Dragon Ball Super introduced the multiverse concept, fanfiction could explore how Goku interacts with and learns from beings from other universes, making him more worldly and less naive in some ways.
One way is to focus on their shared adventures. For example, when they are on a quest for a new power source or to stop a new threat, they can rely on each other more. Goku can protect Bulma physically, while Bulma can use her intelligence to help Goku strategize. Another way is to explore their past more deeply. Maybe there are some untold stories from when they first met that can strengthen their bond.
They could start by sharing more personal stories. For example, Goku could tell Bulma more about his time on Planet Yardrat. This would deepen their understanding of each other.
One idea could be Goku Jr facing a new, powerful enemy that has a unique power related to time manipulation. This would force him to train in ways he never has before and seek help from unexpected allies, like some of the older, wiser characters from the original Dragon Ball Z series.
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.
Obviously, Goku and Goku Jr are the main characters.
In this fanfiction, they might have an epic sparring session. Goku could pass on some of his old fighting techniques to Goku Jr.
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.