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.
Well, a common theme is the exploration of different universes. In gogeta crossover fanfic, the idea of Gogeta traveling to different fictional universes and dealing with their unique challenges is quite popular. It's like he could end up in a universe full of magic and have to figure out how his power works there. Another theme is the combination of fighting styles. Since Gogeta is a powerful fighter, seeing how his fighting style combines with those from other characters in a crossover is exciting for fans. And of course, there's the theme of saving the multiverse. Many fanfics have Gogeta as the key to preventing some sort of cataclysm that affects multiple universes.
Some dedicated Dragon Ball fan communities on social media platforms might also be a good source. For example, certain groups on Facebook or Reddit where fans discuss and share their fanfiction works. However, the availability of exactly this pairing might vary. You may need to do some searching within those communities.
In a crossover with the Naruto universe, Naruto himself could be an ally. Naruto has a way of seeing the good in people, and he might try to reach out to Broly and convince him to use his power for good. His ability to befriend and understand others could be key.