A plotline could involve Broly being sent to Kale's universe by some sort of interdimensional rift. Kale initially sees him as a threat and attacks. But as the fight progresses, they start to understand each other's power and motives. Or perhaps a fanfic could have a story where both Broly and Kale are training on the same planet. Their training methods clash, leading to a fight where they both learn from each other and grow stronger in the process.
A plotline could start with Frieza hearing about Broly's power and being jealous. He then devises a plan to ambush Broly. But Broly, being extremely perceptive in his own way, senses Frieza's approach and is ready for the fight. As the fight progresses, Frieza realizes that Broly is much stronger than he expected and has to use all his dirty tricks to try and win. Or perhaps the plotline could involve Broly being captured by Frieza's forces, and then Broly breaks free and goes on a rampage against Frieza and his minions.
One aspect of Vegito and Female Gogeta fanfiction could be the exploration of their unique power combinations. In these fanfictions, writers might create scenarios where they team up against a powerful enemy. For example, they could face a new, extremely strong villain that neither could defeat alone. Their combined strength and different fighting styles would make for an exciting story.
One common theme is power comparison. Writers often explore how Vegito's power from Dragon Ball stacks up against the powers in the other crossover universe, like the magic in a Harry Potter crossover or the quirks in a My Hero Academia crossover.
Maybe it's because Bulma has her own established character traits and relationships in the original story. She might not fit well into the scenarios often presented in Vegito fanfic. Also, her character is more focused on other aspects like her scientific work and family in the canon, so fanfic ideas involving her in a Vegito - related context could seem out of place to her character.
If Vegito never unfused, it might mean that the enemies he faced were simply not strong enough to force the separation. This could imply that the story's difficulty level was lower than originally thought. Or, it could be that some external factor prevented the unfusing, creating a whole new set of plot twists and complications.