One interesting story could be about a new villain who forces Goku to reach the SSJ4 form in a completely different way. Maybe he has to combine his Saiyan powers with some ancient energy source on a new planet. Another could be a fanfiction where Vegeta finally masters the SSJ4 form before Goku and has to face a powerful foe alone.
In some fanfictions, the SSJ4 form could be linked to a prophecy. The story could be about how Goku and his friends have to figure out the meaning of this prophecy related to SSJ4. It could involve ancient scrolls and wise old characters. They might discover that the SSJ4 form is the key to saving not just Earth but the entire multiverse from a great evil that is slowly spreading across different dimensions.
One common theme is power exploration. Writers often explore the full extent of Goku's SSJ4 power in Dragon Ball Super scenarios. Another is the idea of new battles. Fans like to imagine Goku in SSJ4 facing off against different villains. Also, friendship and family themes are common, like how Goku's new power affects his relationships with his friends and family.
Well, there are some great ones. A lot of fanfiction in this area explores the power of Goku in his SSJ4 form in the context of the Dragon Ball Super universe. Some stories might focus on battles that Goku could have in this form against new and powerful enemies, while others could be about his relationships with other characters while being in this super - powerful state. It really depends on what you're interested in.
Goku is almost always a main character. His journey to master or use the SSJ4 form in new and exciting ways is a common theme. Vegeta is also often included, especially when it comes to rivalries and his own pursuit of power in the form of SSJ4. Sometimes, other Saiyans like Trunks or Goten might also be featured, especially if the story involves the next generation of Saiyans and their relationship with the SSJ4 transformation.
One interesting story could be about a new Saiyan character introduced in the fanfiction. This Saiyan might have a unique power that even Goku and Vegeta are intrigued by. Maybe they train together to unlock new levels of Super Saiyan forms.
One interesting story could be about Caway's hidden powers. Maybe she discovers she has a unique ability that can change the course of a big battle in the Dragon Ball Super universe.
There are fanfictions that focus on the relationships between the characters. For example, a story where Vegeta has to team up with Frieza in an alternate universe to fight a common enemy stronger than both of them. This forces Vegeta to put aside his hatred and work with his old foe, creating an interesting dynamic.
A great fanfiction idea is a fusion of the power systems. For example, a story where characters can combine chakra and ki. Naruto could try to use ki - enhanced rasengan or Goku could use a chakra - based Kamehameha. It would be an exciting exploration of both universes' power concepts.
Another great 'Broly Dragon Ball Super Fanfiction' could revolve around Broly's past on Planet Vegeta. What if there were more secrets about his family? Maybe he had a sister or a brother who was also extremely powerful but was lost during the destruction of the planet. This fanfiction could explore Broly's search for his lost family member while still dealing with the threats in the present universe.
A great fanfiction could be Whis falling in love with a new character. This new character might be a powerful being from a different galaxy. Their relationship would bring a whole new dynamic to the story. Whis would have to balance his duties as an attendant to Beerus with his new - found feelings. Also, there could be a story where Whis creates a new form of training for Vegeta that helps him finally surpass Goku in power, which would be a huge twist in the Dragon Ball Super narrative.
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.