A plot twist could be that Android 18 has some hidden Saiyan DNA. This would completely change the dynamic between her and Vegeta. They might then explore their newfound connection related to their Saiyan heritage together. Maybe she could even unlock some Saiyan - like powers, which would be a huge shock to everyone in the Dragon Ball Z universe.
One possibility is a story where Vegeta tries to train Android 18. He might see her potential and want to push her to become even stronger. This could lead to some interesting training sessions, with Vegeta's harsh Saiyan training methods clashing with Android 18's more laid - back attitude. However, in the end, she could become a stronger fighter under his guidance.
One possible plot could be that Android 21, with her unique powers, teams up with Goku to face a new and powerful enemy. They might have to combine their strength and fighting techniques to defeat this foe.
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.
One idea could be a story where Vegeta and Trunks train together in a new, super - intense way. They could discover a new form that combines their Saiyan powers in a unique way. For example, they might find a way to fuse their ki into a powerful energy blast that can defeat even the toughest enemies.
Well, a plot could be Android 21 somehow getting her hands on the Namekian Dragon Balls. Since she has that scientific mind, she might try to figure out how to modify the wishes. In the process, she could draw the attention of Frieza who wants the Dragon Balls for his own evil purposes. Then the Z - fighters would have to step in to protect Android 21 and the Dragon Balls. Another plot idea is that Android 21 is from an alternate universe where she was created by the good guys. When she crosses over to the main Dragon Ball Z universe, she has to adjust to the different power levels and politics among the characters. And she could end up being a key player in the fight against Buu, perhaps using her unique abilities to weaken Buu in some way.
A possible plot twist could be that Vegeta is actually an impostor. The real Vegeta has been trapped in another realm, and this impostor has been living with Bulma and the others. As Bulma starts to notice some strange behavior, she investigates and discovers the truth. Then she has to find a way to rescue the real Vegeta.
One idea could be that they have to team up in a new and strange way due to a unique threat. Maybe there's a villain who can only be defeated by a special combination of their powers that they discover through their new 'mate' relationship. For example, they might have to combine their ki in a way they've never done before.
One idea could be that Female Goku and Vegeta have to team up to save the universe from a new and powerful enemy. They start off not getting along well, like in the original series, but as they face challenges together, they develop a mutual respect and friendship.
One idea could be a story where Bulma and Vegeta go on a secret adventure together to find a new power source for Capsule Corp. Another could be a plot where they have to deal with an alien threat specifically targeting their family. Maybe a story about how they first fell in love from Vegeta's perspective, including his inner thoughts as he slowly warmed up to Bulma.
One interesting plot could be about Vegeta trying to understand Earth's customs for Bulma's sake. For example, he might struggle with things like birthday parties or family holidays. Another plot could be Bulma inventing a new device that gives Vegeta a new power boost, but it has some unexpected side effects on his Saiyan pride. Or it could be a story where they go on a vacation together to a new planet and face various challenges there.