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.
It's a bit hard to say for sure. Both Vegeta and Goku have their moments of strength. Goku has some unique powers and techniques that give him an edge at times, but Vegeta is no slouch either and constantly works to catch up and surpass Goku.
Generally, Goku still holds the edge over Vegeta. Their power levels fluctuate, but Goku often achieves higher peaks of strength and unlocks more powerful forms first.
It often starts with a shock or surprise at the mpreg situation. Then, they gradually learn to rely on each other more. For example, Vegeta might start to show a more caring side that he usually hides, and Goku might become more responsible. They'll have to communicate more, sharing their fears and hopes about the future with the baby.
There are indeed some. I've come across a couple where the authors beautifully develop their relationship. They often play with the idea of how their different personalities, Goku's adventurous and friendly nature and Fem Broly's more complex, often misunderstood character, could come together in a romantic context. It's interesting to see how different writers approach this unique pairing.
One possible analysis could be that it often explores the dynamic between two powerful and complex characters. Goku Black, with his menacing and somewhat twisted nature, and Vegeta, the proud Saiyan prince. In fanfics, their interactions can range from intense battles to unexpected alliances. Maybe it delves into how Vegeta would react to Goku Black's presence, given their different ideologies and power levels.