Another possibility is that something from their pasts resurfaces. Maybe Broly remembers some harsh treatment he received long ago, and Goku, upon learning about it, feels sad for him. Or Goku's own past traumas are brought up during an encounter with Broly, and Broly, in his own way, feels sad for Goku. This could lead to a very emotional and complex relationship between them in the fanfic.
A fanfic about Broly and Goku being sad could involve many things. Maybe it's about a great battle they had that led to destruction and regret. For example, they might have fought so fiercely that innocent lives were lost, and now they are both sad about the consequences. Or perhaps it could be about a rift in their relationship. Maybe Broly feels misunderstood by Goku, or Goku is disappointed in Broly for some reason.
It could be that in this sad fanfic, one of them is injured badly. Say Goku is seriously hurt, and Broly, who has always seen Goku as a worthy opponent, now feels a sense of loss and sadness. He might be worried that he'll never have that epic fight with Goku again. On the other hand, if Broly is the one hurt, Goku could be sad because he realizes that Broly is also a being with his own emotions and not just a powerful enemy.
It could involve an adventure where they team up to fight a new powerful enemy. Maybe they discover a new form of energy together and have to train to master it.
One common theme could be loneliness. Both Broly and Goku have had their moments of isolation. Goku was alone on Earth as a baby, and Broly has often been an outcast. In a sad fanfic, this loneliness could be emphasized, and they might find some sort of connection through their shared feelings of being alone. Another theme could be powerlessness. Despite their great strength, there could be situations where they feel they can't protect what matters to them, leading to sadness.
Well, it could entail their adventures together. Maybe it focuses on how they first met and started their journey to find the Dragon Balls. Their relationship is quite interesting as they are from different backgrounds, with Goku being a naive but powerful Saiyan and Bulma being a smart and resourceful Earthling.
One common theme is redemption. Since Broly is often portrayed as a villain initially, many fanfics focus on his journey towards redemption with Goku's help. For example, Goku might show Broly a different way of using his power, leading Broly to change his ways.
A common theme is rivalry - turned - friendship. Given their history in the original story, many fics explore how they can move past their animosity.
Well, one possibility is that Goku's friends would immediately start a search and rescue mission. Vegeta, being his rival but also an ally, would probably be at the forefront. He'd be furious that Broly dared to kidnap Goku. They'd likely track Broly's energy signature to wherever he's taken Goku. Another thing could be that Goku, while kidnapped, tries to reason with Broly. Maybe he discovers that Broly has some deeper issues or motives for the kidnapping that could potentially be resolved through conversation.
Perhaps it's a love story where Cheelai's feelings for Broly develop over time. They might start as friends, but as they go through various challenges, her admiration turns into something deeper. It could also explore how Broly, who is often seen as a wild and powerful being, reacts to Cheelai's more gentle and caring nature.
Well, Goku's training is intense. It might involve extreme physical conditioning like his gravity training. Naruto could be pushed to his limits in terms of strength and speed. He'd probably learn about ki control, which is different from chakra but could enhance his abilities. Goku might also teach Naruto about the importance of a strong mental state during battle, something Naruto already has a bit of but could be refined.
Well, they might be really shocked at the power levels of Goku and Broly. RWBY characters are used to their own kind of battles, and seeing the insane energy blasts and transformations in the Goku vs Broly fanfic could leave them in awe. They might start thinking about how they could incorporate some of those powerful moves into their own fighting styles.
A fanfic about Bunnelby and Eevee could be really interesting. It might be about their adventures in the Pokémon world. Maybe they team up to find a rare item or to rescue a captured Pokémon. They could explore new regions together, facing different types of Pokémon and trainers along the way.