Maybe in the fanfiction, Goku and Fem Vegeta start a harem as a way to train stronger fighters. They bring in various characters from different planets and train them in their unique ways. Along the way, romantic feelings start to develop between the members of the harem and Goku and Fem Vegeta have to deal with the resulting jealousies and rivalries.
Android 18 might be a good addition. She's strong and has a complex personality. Her presence could add a new dynamic to the harem, especially with her ability to stand up to both Goku and Fem Vegeta. And her relationship with Krillin could also be a factor in the story.
One idea could be that Goku and Vegeta are transported to an alternate universe where they have to team up with their alternate selves to defeat a powerful new enemy that is a combination of all the foes they've ever faced. This would force them to not only work together but also understand different versions of themselves.
One idea could be that Goku and Vegeta travel back in time to the Saiyan - Planet. They could try to change the fate of the Saiyans, like preventing Frieza from destroying their planet. This would bring a whole new set of challenges as they would have to deal with the Saiyan culture and politics of that time.
A possible plot could involve an evil force that targets Goku and Female Vegeta specifically because of their combined power. They are constantly on the run and have to rely on each other. In the process, they discover that their ki energies can be combined in a unique way. This discovery not only helps them fight the enemy but also deepens their bond as they realize how well they complement each other.
One interesting plotline could be Goku and Fem Vegeta teaming up for a special mission in a new, unknown part of the universe. They might face challenges that force them to rely on each other's unique skills and grow closer as a result.
One idea could be that Goku is transported to a different universe where he meets various female characters from different anime. They are all intrigued by his power and his pure-hearted nature, and a harem dynamic starts to form as they all want to be around him and train with him.
One idea could be Goku from Dragon Ball crossing over into a world like Fairy Tail and having a harem of the female mages there. Maybe he helps them in battles and gradually forms relationships with them.
Well, a possible plot could be that Goku and Vegeta are in different high school cliques. Goku is with the friendly and outgoing group, and Vegeta is in the more serious and studious clique. However, a school event forces them to interact. They initially clash due to their different personalities and ways of doing things. But then, they find out they both have a shared goal of protecting the school from a hidden threat. This makes them put aside their differences and combine their powers. Or perhaps they both have a crush on the same girl in high school, and they compete in a friendly way to win her heart, which also leads to some character growth for both of them.
Perhaps a fanfiction where Vegeta and Goku get transported to an alternate universe. In this universe, their roles are reversed, and they have to adapt to the new situation while still trying to find a way back home. They would face challenges that are unique to this new world and learn from each other in different ways.