Well, perhaps the God of Destruction has a premonition of a great threat that only Goku can help with. So he seeks out Goku. Goku, being Goku, is eager for a new challenge. As they join forces, Goku's friends are skeptical because they don't trust the God of Destruction. However, Goku's positive influence starts to change the God of Destruction's attitude towards mortals, and they end up saving the multiverse together.
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.
A plot could involve Goku being accidentally transported to the realm of the God of Destruction. There, he is seen as an intruder at first. But as he shows his skills and his pure love for battle, the God of Destruction decides to train Goku. Goku, in turn, teaches the God of Destruction about the Saiyan spirit, and together they face a common enemy that is emerging due to a rift between their two worlds.
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.
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.
Goku becoming a God of Destruction could mean a new look for him. Maybe his Saiyan armor would transform into something more fitting for a god, with a more regal and powerful appearance. His power level would skyrocket, and he could start to have an influence on the balance of the universe. He'd also likely have to deal with other gods, which would lead to some interesting interactions.
One possible plot could be that Goku somehow gets transported to the Bleach universe. He might initially be confused by the concept of Hollows and Soul Reapers. But then, with his power, he could start fighting Hollows, thinking they are some sort of new villains. His ki-based attacks would be a new and powerful force in the Bleach world, and he could end up teaming up with Ichigo to fight a common, extremely powerful enemy, like a super - powerful Hollow that even the Soul Reapers are having trouble with.
One possible plot could be Goku from the SSJ4 form crossing over into the world of Dragon Ball GT. He could encounter new threats that require his ultimate form to defeat. Maybe he teams up with the other Z - fighters in that universe to face off against a powerful and ancient evil that was sealed away long ago and has now been unleashed.
A plot could involve a tournament where fighters from both universes participate. Goku enters because he loves a good fight, and Naruto joins to prove himself outside of the ninja world. At first, there's some misunderstanding between the two groups, but Goku and Naruto quickly realize they have a lot in common. They end up fighting side by side against some of the more underhanded competitors. As they progress through the tournament, they learn from each other's techniques and grow stronger. In the final battle, they face off against a super - strong opponent created by an evil wizard who wants to take over both universes.
One possible plot could be that Lemon, with her unique abilities, somehow gets transported to the Dragon Ball universe. Goku, being the curious and friendly Saiyan he is, decides to help her find her way back. Along the way, they face some common enemies. Goku teaches Lemon about ki and fighting techniques, and Lemon shows Goku some of her own skills from her world. This could lead to an epic team - up against a powerful villain who threatens both their worlds.
One possible plot could be that Goku accidentally travels to a world where Lemon is a powerful being. They start as rivals but gradually learn from each other's strengths. Goku could teach Lemon about his fighting techniques and his Saiyan heritage, while Lemon could show Goku some unique powers or skills from their world. Then they team up to face a common enemy that threatens both of their worlds.
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.