Good character development in a 'Reborn as Goku' fanfiction also involves relationships. The character should interact with the other Dragon Ball characters in a believable way. They might have different reactions to Goku's friends and family compared to how Goku himself would. For instance, they could be more suspicious of Vegeta at first due to their past experiences. As they form bonds with these characters, it should show growth in their personality. They could learn loyalty from Goku's friends and become more selfless in the process.
One common plot twist could be that the reborn character has some unique abilities from their previous life that they can combine with Goku's powers. For example, they might have a special form of magic that they can use in combat along with Goku's martial arts. Another could be that they are reborn not just as Goku, but with a hidden mission that even Goku himself is unaware of in the original story. This could lead to some interesting interactions with the other Z - fighters as they try to figure out what's going on.
One interesting plot point could be Goku's combat skills being transferred to Naruto. For example, Goku's unique fighting styles like the Kamehameha might be reimagined in the Naruto universe. Another could be how Goku's Saiyan nature, such as his love for battle and quick growth in power, affects Naruto's character development. Maybe Naruto starts to have an even greater appetite for tough fights and his power - up speed becomes even more astonishing.
When Goku is reborn as Naruto, his personality could shift in several ways. His naivete might be toned down a bit as he has to adapt to the more nuanced world of Naruto. He may become more empathetic towards the villagers' plights since Naruto has his own backstory of being an outcast. However, his love for challenges and growth in power would still be there, but now it would be more intertwined with Naruto's goals of protecting the village and achieving peace in the ninja world.
Well, Vegeta would be a likely character. He might be initially suspicious of the female Goku. Maybe he thinks she's a threat or just can't believe there's another Goku - like being. He could be there to add some comical moments as he tries to understand the situation.
In general, it's a bit of a toss-up. The strength of Manga Goku and Anime Goku can vary depending on the specific arcs and battles. Sometimes, it might seem like one is stronger in a particular situation, but that could change in another.
Another possible main character could be the villain that forces the two Gokus to meet. This villain could be a new, super - powerful threat that the two Gokus need to deal with. Their meeting becomes a crucial part of the story to defeat this powerful foe.
One of the characters who could betray Goku is Piccolo. Although they've been allies for a long time, in fanfiction, things can change. Perhaps an ancient evil force takes control of Piccolo's mind and makes him turn on Goku. Or it could be Krillin. He might be influenced by an outside force that convinces him Goku is becoming too dangerous. Also, don't forget about the Kais. If there's some sort of power imbalance or a threat they perceive from Goku, they might betray him to maintain the order of the universe in the fanfiction.