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.
One idea could be Goku struggling to adapt to the role of God of Destruction. He's used to being a protector, so having to destroy planets as part of his new duty would create a great internal conflict.
One interesting 'God of Destruction Goku fanfiction' could be where Goku takes on the role of a God of Destruction in a different universe. He might have to deal with new threats and use his Saiyan powers in unique ways. For example, he could face off against an enemy that can manipulate time and space, and Goku has to find a way to overcome this using his God of Destruction powers.
Well, in 'god of destruction crossover fanfiction', a cool concept is the cultural exchange. If the 'god of destruction' from one universe crosses into another, say a medieval fantasy world, there could be a really interesting mix of cultures. The god of destruction might introduce new concepts of power and divinity that the fantasy world has never seen before. Also, the idea of redemption or change in the god of destruction's nature can be fascinating. Maybe in the crossover, they start to question their role as a destroyer due to the influence of the new world they're in.
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.
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.
One interesting concept could be how Goku adapts his martial arts skills to the gaming world. Maybe he uses his instant transmission in virtual battles in a really cool way.
One possible plot could be Goku, in his Saiyan tenacity, challenges the God of Destruction to a friendly spar. The God of Destruction initially scoffs but is soon impressed by Goku's power. As they fight, their powers start to blend, creating new energy forms that threaten to disrupt the balance of their universes. They then have to team up to contain this new energy while also learning from each other's fighting styles.
One interesting concept could be new power - ups for Goku. Maybe he discovers a unique form during the Cell Games in the fanfic.
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.
In fanfiction about Goku and Goku Black, a common concept is the alteration of their power levels. Writers may give Goku new forms or boost Goku Black's abilities in unexpected ways. There can be stories centered around Goku Black's adaptation to the main timeline's culture and society. For example, he might have to deal with the everyday life in addition to his evil plans. And there are fanfictions that focus on the psychological impact of their battles on each other, like how Goku might be affected by constantly facing an evil version of himself.