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 many fanfics, Broly would likely win. Broly is known for his extreme power and near - uncontrollable rage which gives him a huge strength boost. Frieza, while powerful, has been shown to be outmatched by the likes of Goku and Vegeta. And Broly is often depicted as even more powerful than them in some fan interpretations. So, in a one - on - one fight in a fanfic, Broly has a good chance of coming out on top.
Frieza might initially be shocked. He's used to being a feared and powerful being, and the idea of having a family is very foreign to him. So he could be in denial at first.
For Yamcha, you can focus on his growth from being a bit of a comical character to a more serious and respected fighter in the context of his relationship with Frieza. Maybe he learns from Frieza's cold and calculating nature how to better strategize in battles. For Frieza, his development could be about him learning to care for someone other than himself. He could start to see Yamcha as an equal, which is a huge shift from his usual superiority complex.
If Broly participated in the Tournament of Power, he would be an absolute powerhouse. His raw strength and uncontrollable rage could either make him a wild card that disrupts the whole tournament or a key player for his team. He might start by randomly attacking opponents without much strategy, but as the fight goes on, his natural fighting instincts could kick in and he could become a real threat to the strongest fighters in the multiverse.
Bulma could be another character paired with the reborn Broly. She might be the one to help him understand the modern world if he is reborn in a different time period. She could use her intelligence to create devices that help Broly control his power better or she could just be there to provide some comic relief in the midst of all the intense Saiyan drama.