Well, his relationship with Vegeta would be interesting. If Goku went back in time and showed Vegeta the SSJ God form too early, Vegeta might become more competitive or even jealous earlier. He might push himself even harder to catch up, which could lead to more intense rivalries and also more joint training sessions between them.
Goku might run into past versions of his friends. He could see a younger Krillin or Bulma and have some really interesting interactions. He could also meet past versions of his enemies and try to change their fates. For instance, he could meet a young Vegeta and try to steer him away from the path of evil. This would be a huge change in the Saiyan's story.
Goku being born with SSJ4 in fanfic would likely make him a force to be reckoned with from infancy. This could result in quicker power-ups, different battles, and maybe even a shift in the balance of power among the characters.
One idea could be Goku traveling back in time to train with his past self. He could teach his younger self how to achieve the SSJ God form more quickly, which would change the power dynamics in his original timeline battles. For example, against Frieza, if he had the SSJ God power earlier, the fight would be much shorter and less arduous.
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.
In Dragon Ball fanfic, Goku SSJ often has super strength, like being able to lift extremely heavy objects with ease. He can also move at incredible speeds, making it hard for enemies to even see him coming. His energy blasts are much more powerful than in his normal form, capable of destroying large structures or powerful foes in one shot.
The plot might involve Goku SSJ4 teaming up with some unexpected allies in the Tournament of Power. He could be using his SSJ4 powers in creative ways, like combining them with the techniques of his new allies to gain an edge over the other universes' competitors.
In some fanfics, Goku SSJ's character development might focus on his relationship with his family. He could become more protective and understanding of his family's needs. For example, he might take more time to train Gohan and teach him important life lessons along with martial arts skills.
Vegeta would be a great character to include. His rivalry with Goku could add some interesting dynamics in the crossover. Also, Piccolo, with his strategic mind and powerful attacks, could be useful in facing the new challenges. And if it's a crossover into another anime universe, someone like Naruto could be included. Naruto's never - give - up attitude and his own set of powerful jutsu could create some amazing team - up moments with Goku.