Power and training. Since Goku is known for his power - seeking, his brother in fanfics often has a similar drive. They might train in unique ways or discover new forms of power.
A really interesting 'brother of Goku fanfic' is one where the brother was sent to a different planet. The story details his journey to find his way back to Earth and reunite with Goku, while also facing many powerful enemies along the way.
One possible plot could be that Naruto, as Goku's brother, has to learn the ways of ki while also maintaining his ninja skills. Maybe they team up to fight a common enemy that threatens both the ninja world and the Dragon Ball universe.
If Goku shared spirit with his brother in a fanfic, it could potentially change the dynamics of their relationship and unlock hidden powers. It could also create new challenges and conflicts that they have to overcome together.
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.