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.
Maybe in the fanfiction, Goku and Vegeta accidentally swap powers. Goku has to deal with Vegeta's more calculated and strategic fighting style with his new Galick Gun - like abilities, while Vegeta has to adapt to Goku's carefree and instinct - based fighting using the Kamehameha. It would be a humorous and action - packed plot as they both struggle to master their new powers while still being themselves.
A great plot could be that a powerful wizard from another dimension casts a spell on Goku and Vegeta, making them relive their most difficult battles but with a twist. They have to fight these battles together as a team instead of separately. This would require them to communicate and strategize, which is not something they are used to, especially Vegeta who has always been more of a lone fighter at times.
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.
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.
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.
One idea could be a tournament where Goku and Naruto are both invited. They start off as rivals but then realize they have more in common as they face powerful opponents together. Another could be a situation where an enemy from Goku's universe somehow ends up in Naruto's world and they have to team up to defeat it. A third idea is that a magic spell or some kind of power - related accident causes their powers to be swapped, leading to comical and also challenging situations as they try to adapt to each other's abilities.
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.
Another plot could involve Xeno Goku getting stranded in a different anime world during the crossover. He has to adapt to the new rules and powers there while also trying to find a way back home. He could make new friends and enemies in this strange world.
One idea could be Goku crossing over into a different anime universe, like the Naruto world. He could end up in a fight tournament there and show off his Super Saiyan Blue form. His ki attacks would be a new and strange concept to the Naruto characters.