Another reason could be that Bulma thinks Chi - Chi is holding Goku back from reaching his full potential as a Saiyan. Goku is a powerful fighter, and Bulma might believe that Chi - Chi's more traditional family values and her desire for Goku to be a family man are preventing him from training as much as he should or from going on important missions to protect the Earth.
One possible motive could be that Bulma is worried about Goku's safety. Since he's constantly getting into dangerous battles, she might think that if he's not tied down to Chi - Chi, he could be more focused on self - preservation. Maybe she feels that Chi - Chi's family demands on Goku are a distraction and could lead to his downfall in a fight.
Bulma could be jealous. Not in a romantic way towards Goku, but perhaps she's jealous of the relationship Chi - Chi has with Goku. Bulma has been friends with Goku for a long time and might feel that Chi - Chi is taking Goku away from their adventures and friendship. She might want to break them up so things can go back to how they were before Goku got married.
Goku would probably be really confused. He loves Chi - Chi and their family, and he might not understand why Bulma would want to break them up. He might just tell Bulma straight that he's happy with his life as it is. Chi - Chi, on the other hand, would be furious. She's very protective of her family and her relationship with Goku, and she'd likely give Bulma a piece of her mind.
One possible plot could be that Bulma might be jealous of the relationship Goku and Chi - Chi have. Maybe she sees how much Goku has changed since being with Chi - Chi and she misses the old, carefree Goku. So, she tries to create misunderstandings between them by, for example, making it seem like Goku is more interested in training with her than spending time with Chi - Chi.
In a fanfic, the Bulma - Chichi fusion could be part of a tournament arc. They enter the tournament fused and shock everyone with their abilities. Their opponents underestimate them at first, thinking it's some kind of strange trick. But as the battles progress, they show that their fusion is not only powerful but also has a deep understanding of both martial arts and strategic thinking.
Well, the plot could involve them fusing due to some kind of magical or technological accident. Maybe they are trying to defeat a powerful enemy together and their combined energy causes them to fuse. They could then have a new form with combined powers and a unique personality that is a blend of their own.
One possible plot could be that Bulma and Chichi team up for a scientific - martial arts tournament. Bulma uses her intelligence to create unique gadgets for Chichi to use in the combat part of the tournament. This brings them closer as they train together and face various opponents.
It could also be due to the way the fanfic writers handle their characters. If they write Bulma as a prideful character and Chichi as someone who challenges that pride, either directly or indirectly, Bulma might be written as hating Chichi. Fanfic often takes creative liberties and this could be one of the results.
Chi - Chi would likely be very hurt at first. But then she would get angry and start thinking of ways to prove to Goku that their relationship is strong. She might cook his favorite meals and remind him of all the good times they've had together, while also giving Bulma a piece of her mind.
One common theme is friendship. Since they are both important female characters in the Dragon Ball universe, fanfics often explore how they become friends and support each other. For example, they might team up to deal with some threats in the story.
Well, if they fuse, the new character would likely have Bulma's inquisitive nature. They would be constantly curious about new things, especially in the realm of science and technology. At the same time, they would also possess Chichi's determination. So when facing challenges, they would be very persistent.
It could be about an alternate universe where Goku and Bulma's relationship takes a romantic turn. Maybe in this fanfic, they face challenges that are more complex and mature - themed, like dealing with power dynamics in a different way compared to the original series. The Rated M aspect might mean there are some more intense emotional or even physical interactions that are not typical of the regular Dragon Ball story.