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.
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.
Well, Vegeta would be a likely character. He might be initially suspicious of the female Goku. Maybe he thinks she's a threat or just can't believe there's another Goku - like being. He could be there to add some comical moments as he tries to understand the situation.
In general, it's a bit of a toss-up. The strength of Manga Goku and Anime Goku can vary depending on the specific arcs and battles. Sometimes, it might seem like one is stronger in a particular situation, but that could change in another.
Another possible main character could be the villain that forces the two Gokus to meet. This villain could be a new, super - powerful threat that the two Gokus need to deal with. Their meeting becomes a crucial part of the story to defeat this powerful foe.
One of the characters who could betray Goku is Piccolo. Although they've been allies for a long time, in fanfiction, things can change. Perhaps an ancient evil force takes control of Piccolo's mind and makes him turn on Goku. Or it could be Krillin. He might be influenced by an outside force that convinces him Goku is becoming too dangerous. Also, don't forget about the Kais. If there's some sort of power imbalance or a threat they perceive from Goku, they might betray him to maintain the order of the universe in the fanfiction.
Well, it could be about Goku Jr learning some of Goku's most powerful moves. Maybe Goku teaches him the Kamehameha. They might also go on an adventure together, like fighting a new and powerful enemy that neither could face alone.