One common theme is the clash of cultures. Harry Potter's world is full of magic based on spells, potions and wands, while Broly comes from a world of physical strength and power - ups. So, seeing how these two different power systems interact is a big theme.
Some fan fictions may put Jaiden in a crossover scenario. Like she enters the world of a popular video game and has to adapt to the game's rules and characters. She could use her knowledge of animation to outwit the game's villains and make new friends among the game's inhabitants.
There's a fan fiction titled 'Broly in Hogwarts'. In this, Broly somehow gets transported to Hogwarts. It details his confusion at first, as he's used to his own universe. But then he starts to adapt and uses his strength to help the students when the school is in danger. It's really exciting as it shows the clash and then the cooperation between two very different universes.
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.
He could team up with Luffy. Luffy is always open to new allies, and Broly's power would be a great addition to the Straw Hat crew. They could go on adventures together and face powerful enemies.
Goku could be a likely ally. Despite their initial conflicts, Goku has a way of seeing the good in people. He might approach Broly and convince him that they can work together to win the Tournament of Power for their universe. Their combined strength would be a formidable force.
In a crossover with One Piece, Luffy could be an ally. Luffy is known for his open - mindedness and ability to befriend almost anyone. He would see Broly's power and might try to get him to join his crew or at least fight alongside him against common enemies. They could have some really fun interactions, like Luffy trying to make Broly laugh or share his love for food with him.