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.
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.
Since Broly has a rather wild and untamed personality, Naruto could learn to be more in touch with his primal instincts. This doesn't mean Naruto would become a brute, but rather he could find a new balance between his kind - hearted nature and a more aggressive side when necessary. For example, in negotiations or battles, Naruto could be more intimidating. Also, Broly's loyalty to his own beliefs (even if they are sometimes extreme) could inspire Naruto to be more firm in his own ninja way.
If Broly joined the Tournament of Power, it would be an absolute game - changer. His raw power is off the charts. He could easily take on multiple opponents at once. He might start by targeting the strongest foes, like Jiren. His uncontrollable rage could work in his favor, as opponents would be intimidated. However, his lack of teamwork skills could also be a drawback. But with his power, he could probably just brute - force his way through many battles.
Yes, there are likely fanfictions out there with the concept of 'naruto is broly'. Fanfiction writers often come up with creative crossovers like this. You can try searching on popular fanfiction websites such as FanFiction.net or Archive of Our Own.