One possible plotline could be that Cell, seeking to prove his superiority, challenges Broly. Broly, in his usual berserk state, accepts without hesitation. The fight could start with Cell trying out his various attacks like the Kamehameha - like Solar Flare and his physical strikes. But Broly's power is just too much and he starts to overpower Cell. However, Cell then reveals a new, hidden ability that he has been developing in secret and the tables are turned for a while.
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.
In a fanfic, Naruto might power up by having a spiritual awakening. He could connect with the spirits of past Hokages in a deeper way, gaining their knowledge and some of their power. This could enhance his existing jutsu and give him new abilities. Vegeta could power up if he were to face a near - death experience against Naruto. This could trigger a hidden potential within him, like a new type of Super Saiyan form that has unique abilities. He could also fuse with another Saiyan (if there were another Saiyan available in the fanfic's scenario) to create an even more powerful being.
Well, it's really hard to say. The Black Swordsman has his own set of skills with the sword and his combat experience. But Dark God Vegeta is extremely powerful with his Saiyan abilities. If they were to fight, it would depend on the context. If it's in a close - quarters combat, the Black Swordsman's sword skills might give him an edge initially. However, Vegeta could quickly adapt and use his energy blasts and super - strength to overpower. In a more open area, Vegeta's long - range attacks would be a big threat to the Black Swordsman.
One interesting plot point could be the initial encounter between the two. How would they react to each other? The Black Swordsman might be cautious seeing such a powerful being as Dark God Vegeta. Another point could be the discovery of each other's weaknesses. Maybe the Black Swordsman finds a way to temporarily nullify Vegeta's energy attacks, or Vegeta discovers that the swordsman's sword has a special power that can harm him. Also, the moment when they start to respect each other's fighting abilities during the battle would be really engaging.