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.
In db super crossover fanfiction, the concept of power scaling across different fictional worlds is really engaging. Imagine how the power levels of Dragon Ball Super characters compare to those in, say, the Star Wars universe. Writers often explore how these differences would play out in battles or alliances. Another aspect is the cultural clash. When characters from DB Super enter a different fictional world, their Saiyan or Namekian cultures might interact in unique ways with the new environment's culture.
A simple fanfiction idea could be that Broly and Cheelai start training together to become even stronger. They aim to be the top protectors in their part of the universe.
Goku is definitely one of the most popular. His powerful and adventurous nature makes him a great fit for crossovers. He can be paired with all kinds of characters from other series. Vegeta is also quite popular. His pride and combat skills can lead to some really interesting interactions in crossovers.