Some fanfictions might give Goku a power - up related to the Dragon Balls themselves. For instance, he could gain the ability to manipulate the Dragon Balls' energy in a way that gives him a boost in combat. This could be a unique power - up that combines his connection to the Dragon Balls and his fighting skills.
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.
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.
In this fanfiction scenario, it could be an extremely emotional encounter. Goku, who has grown so much in Dragon Ball Super, finally meeting his father Bardock. Maybe Bardock gets to see how his son has not only become a powerful Saiyan but also a protector of Earth. It could lead to some interesting exchanges about Saiyan heritage and the future of the Saiyan race.
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.
One possible scenario could be Goku and Bulma going on a quest together to find a new type of Dragon Ball that has unique powers. They might face various challenges along the way, like dealing with new villains who also want the Dragon Ball. Bulma could use her intelligence to outwit the enemies while Goku uses his strength to fight them off when necessary.
The world - building aspect is really cool too. In some fanfictions, they create a new hybrid universe where the technology of Power Rangers and the magic/ki of Dragon Ball coexist. This can lead to things like Power Ranger suits enhanced with Dragon Ball technology or vice versa. It gives a fresh take on both franchises and allows for a lot of creativity in terms of how the two different aesthetics can blend together.
Goku is, without a doubt, one of the most popular. His constant pursuit of strength and his ability to overcome seemingly impossible odds make him a favorite for fanfiction writers. They can create all kinds of new adventures for him, like facing off against cosmic threats even greater than what he's encountered in the shows.
A being from a parallel Dragon Ball universe where the power levels are exponentially higher. This enemy would have a completely different set of techniques and abilities. For example, they could control the very concept of energy, not just ki, which would force Goku to adapt his fighting style in ways he's never had to before.
One possible fanfiction idea could be that Goku's daughter has unique powers that are a combination of his Saiyan strength and something new, like the ability to control time on a small scale. She might struggle to fit in with the other Z - fighters at first because they see her as just Goku's daughter, but she proves herself by facing a new, powerful enemy that the others can't defeat easily.
One idea could be a story where Goku and Vegeta from Dragon Ball Super end up in the world of Dragon Ball GT by some sort of time - space rift. They then have to team up with the GT versions of themselves to fight a new, powerful enemy that is threatening both timelines.
One interesting fan theory is about the potential hidden powers of some minor characters. For example, some think that Yajirobe might have a secret super form that was never revealed in the original series. It's just a fun idea that fans like to explore in fanfiction.