One interesting Grand Priest fanfiction could be one where he has a secret past that is slowly revealed. Maybe he was once a regular Saiyan who was chosen by the gods for a special purpose. This could lead to a whole new perspective on his character and his actions in the Dragon Ball universe.
There is a fanfiction that explores the Grand Priest's origin. It details how he came to be in his position of power. It involves a lot of lore from different parts of the Dragon Ball universe and ties them together in a really interesting way. It's a long read but well worth it if you're interested in the character.
It could be another powerful being from outside the known Dragon Ball universe. Maybe a being from a parallel dimension who sees the Grand Priest as a threat to his own plans for universal domination. Another option could be a rogue Angel who has turned against the Grand Priest and the current order of things in the multiverse.
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.
One idea could be exploring the Grand Priest's past. Maybe he had a different role in the universe long ago. Another could be a story where the Grand Priest has to make a difficult decision that goes against the will of the Omni - King. Or perhaps a story where the Grand Priest trains a new character to become a powerful being in the Dragon Ball universe.
In fanfiction, if Goku trains with Grand Priest, he might achieve new levels of power. Goku is always eager to get stronger, and the Grand Priest is extremely powerful. He could learn new techniques that are beyond the Saiyan's current understanding, perhaps even something related to the Angels' unique powers.
Claudia is not a character from the original Dragon Ball series. Goku is the main protagonist of Dragon Ball. He is a Saiyan sent to Earth as a baby. He is known for his powerful fighting abilities, his love for martial arts, and his kind heart. He has saved the Earth multiple times from various threats.
Goku could develop a deeper understanding of the power he wields. The Grand Priest's training would not only focus on physical strength but also on the spiritual aspect of ki. This could lead to Goku having a more profound connection with his Saiyan heritage and the energy that flows within him. He may also become more patient, as the Grand Priest's training methods might be complex and require time to master.
There's a fanfic called 'Goku's Hidden Power'. In this story, Goku discovers a new level of power within himself that even the Kais didn't know about. It delves deep into Goku's character, exploring how he copes with this newfound strength and the responsibility that comes with it. The action scenes are also very well - written.