The plot could start with Majin Vegeta's pride getting in the way. He might feel that Bulma is disrespecting his new power as Majin Vegeta. So, he challenges her to a fight. Bulma initially panics but then decides to stand her ground. She uses her knowledge of Saiyan physiology (from all the time she's spent with Vegeta and the other Saiyans) to find his weak points. However, she also tries to reach out to the real Vegeta inside. As the fight progresses, Vegeta starts to see how strong - willed Bulma is and slowly begins to break free from the Majin influence.
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 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.
One key theme could be redemption. Majin Vegeta is in a complex state, and the fanfic might explore his journey towards redemption through his relationship with Bulma. Another theme could be love's power. It can show how Bulma's love for Vegeta helps him in his struggle.
Well, if they were brothers in a fanfic, it could change how they interact with the other characters. Maybe they'd have a deeper understanding and fewer power struggles between them. Also, their family background and upbringing could be explored in new and interesting ways.