It could be that female vegeta has been training in a secret location with a unique training regime. This gives her an edge over goku in combat. For instance, she might have trained with some ancient master who taught her a special form of ki control that goku has not mastered yet. So when they face off, she is able to use this advantage to defeat him.
I'm not sure of a specific 'female vegeta beats goku fanfic' off - hand. You could try searching on fanfic websites like Archive of Our Own or FanFiction.net. They have a vast collection of Dragon Ball fanfics and you might find one that meets this criteria there.
One reason could be to create a huge plot twist. It's a very unexpected and controversial idea that can immediately grab the reader's attention. Another reason might be to explore different emotional states. For example, showing how Vegeta would react to such a betrayal and how Goku would deal with the moral implications of being involved in such a situation.
It could lead to them becoming an almost invincible force, easily defeating most enemies.
It could lead to some major power-ups for Goku. Maybe he'd learn new fighting techniques and strategies from Vegeta's unique style.
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.
A cool plot could be that an ancient evil force has the power to nullify all Saiyan powers except for Goku and Vegeta's. They are the last hope. However, the evil force has also trapped all their friends and family. So, not only do they have to fight this powerful enemy, but they also have to rescue their loved ones. Along the way, they discover new forms of their Saiyan powers that are not based on anger but on unity and friendship.
The plot could start with Vegeta's sister being sent on a mission to Earth. Goku, being the protector of Earth, initially sees her as a threat. But as they interact more, they realize they have a lot in common. They start to explore their feelings for each other secretly. However, Vegeta discovers their relationship and is furious. He challenges Goku to a fight, thinking Goku has betrayed him. In the end, Goku and Vegeta's sister have to prove to Vegeta that their love is real and that it won't harm their relationship as comrades.
One possible plot could be that Goku is under some sort of mind - control and ends up abusing Vegeta. Another could be in a dark, alternate universe where Goku's character has been twisted and he sees Vegeta as a threat to be eliminated rather than a rival. Maybe there could also be a plot where Goku is tricked into believing Vegeta has done something unforgivable and so he takes out his anger on him.
One common theme is rivalry - turned - friendship. Their long - standing rivalry in the original series is often explored in fanfics as a starting point that then evolves into a deeper connection.
Well, in a 'Vegeta x Goku' fanfic, interesting elements often include new power - up scenarios for both of them. For example, they could discover a new form together while training. Also, when the fanfic delves into their personal lives and how they interact outside of fighting, like Vegeta's family life and Goku's simple - minded yet pure hearted nature, it becomes really interesting. It's about seeing these two iconic characters in a different light and exploring their relationship from different angles.