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.
Perhaps a fanfiction where Vados senses a new threat approaching the universe. She enlists Goku to help her find a way to stop it. Along the way, they encounter strange new races and powers. Goku's simple but powerful approach to fighting combined with Vados' wisdom would make for an interesting dynamic as they search for a solution to this impending doom.
According to my current knowledge, there is no official sequels or extras for the God and God of the Dragon Ball. Although some people claimed to have seen a work similar to the sequels, the officials did not give any relevant information or confirmation. If you want to know more about the Dragon Ball God and God, it is recommended to read related novels or comics, or consult the author or bookstore.
Well, one way is to look at different species in the Dragon Ball universe. For example, take the Saiyan and mix it with some other unique traits. Maybe combine Saiyan genes with the Namekian ability to regenerate. Writers could also consider environmental factors that might cause a mutation. What if Goku was exposed to some super-rare energy during a battle in a far - off planet? Another idea is to play with magic or ancient powers. If there's a long - forgotten spell that alters his genetic makeup in a strange way, that could lead to a really cool mutation.
One of the interesting power - ups could be his Ultra Instinct form. In fanfiction, it might be further enhanced, like having different levels that are not shown in the original. For example, he could have an Ultra Instinct - Kaioken combination which would be extremely powerful.
Caulifla and Goku's relationship in fanfiction could be really interesting. Maybe in this fanfiction, they start as rivals. Caulifla, with her Saiyan pride, wants to prove she's stronger than Goku. But as they fight more, they begin to understand each other's fighting styles and develop a mutual respect. This could lead to some really cool training sessions where they both learn new techniques from one another.
A fanfiction might explore the idea of Bardock seeing Goku's life through a time - space rift. He witnesses all of Goku's battles and the friends he makes. Bardock is filled with pride but also realizes that Goku has a different path than the traditional Saiyan way. This makes him question the Saiyan culture he once knew. Meanwhile, Goku starts to have strange dreams about his father, which leads him to search for more about his Saiyan heritage.
One possible fanfic could be about Bulma and Young Goku's first adventure together. It might start with Bulma recruiting Goku to find the Dragon Balls. They could face various challenges like wild animals and bad guys along the way. Their different personalities, with Bulma being smart and resourceful and Goku being naive but strong, would create interesting interactions.