Obviously, Goku is one of the main characters. He is the center of the story as he takes on the role of a God of Destruction.
There might be other Z - fighters like Vegeta. Vegeta could have an interesting reaction to Goku's new status. He might be jealous or try to also reach a similar level. Another possible main character could be Beerus, the original God of Destruction. He could play a mentor or an antagonist role in relation to Goku in the fanfic.
It's possible that new characters are introduced in the 'Goku God of Destruction fanfic' as well. Maybe there are other Gods of Destruction from different universes who come to interact with Goku. Or some new, original characters created by the fanfic author who are either allies or foes to Goku in his new role.
Vegeta would be a great addition. His rivalry with Goku and his pride as a Saiyan would add an interesting dynamic. He might be initially jealous that Goku is getting to fight and train with the God of Destruction. But then he would also want to prove himself and join in the action. Whis could also be included. His knowledge and calm demeanor could help in situations where the power of Goku and the God of Destruction gets out of hand.
Vegeta could be a great rival. He might see Goku's new position as God of Destruction as another thing Goku has achieved that he hasn't. This could fuel their competitive nature even more.
One interesting concept could be Goku's transformation into a God of Destruction - like form. It might involve new powers and a different way of handling battles.
That really depends on the specific fanfic. It could be a group of heroes, like a team of superheroes trying to prevent the destruction. Maybe it's a lone protagonist who has some special ability or knowledge that can stop the countdown.
Goku does reach a state of immense power in the manga that rivals that of a God of Destruction. However, the specific title and all its implications might not be straightforwardly assigned to him. It's a complex development within the story.
There could also be original characters created specifically for this fanfic. Maybe a game master who controls all the virtual worlds Goku enters. This character could be a bit of a mystery, with their own motives and backstory, and they interact with Goku in ways that drive the plot forward.
Often, there are also characters who oppose the OC god of destruction. They might be heroes or protectors of the realms that the OC is set to destroy. These characters will try to stop the OC from causing destruction, leading to many conflicts and battles throughout the fanfiction. And there could be some side characters, like the friends or servants of the main characters, who add more depth and complexity to the story.
Obviously, the OC (Original Character) who is the God of Destruction is one of the main characters. There might also be some of the well - known DBZ characters like Goku or Vegeta, who could either be allies or antagonists depending on the plot.