Dragon Ball Super: Broly, which saw Goku and Vegeta team up to take down the seemingly unstoppable son of an exiled Saiyan.
Broly is the most powerful Saiyan ever to appear in the world of Dragon Ball, as neither Goku or Vegeta could beat him independently. Even teaming up and going Super Saiyan Blue wasn't enough to stop Broly, who became even more dangerous when Frieza killed his father. This sent him into an uncontrollable rampage. Broly, a villain who is characterized by his fits of rage, couldn't be reasoned with and couldn't be stopped until Goku and Vegeta fused to become Super Saiyan Blue Gogeta. Once he was beaten, Broly returned to his base form where he was able to maintain a docile state. Goku visited him at the end of the movie and said he looked forward to facing him again.
Dragon Ball Super: Broly leaves the door open for Broly to return, but is it possible for him to ever be used as a member of the Z-Warriors? Broly can't seem to power up without going berserk. Dragon Ball Super's Tournament of Power arc reveals how the next anime can fix this problem. During the tournament, Goku's Universe 7 team discovered that Universe 6 has its own Legendary Super Saiyan in the form of Kale, who goes from a timid Saiyan to a force of destruction. For a while, Kale was a wild card who attacked anyone around her when she was in her Legendary Super Saiyan form, but a team-up with Caulifla against Goku taught her how to control her power. At Caulifla's urging, Kale was able to calm herself down and enter into a slightly slimmed-down variation of this transformation, which has been dubbed "True Legendary Super Saiyan."
The benefit of this ability is that it allowed Kale to keep the power she had as a Legendary Super Saiyan without sacrificing her self-control. Now, Kale can fully power up without her allies worrying that she'll turn on them. What this means is that a Legendary Super Saiyan can be a hero. The franchise going in this direction with Broly would make sense, based on what it's already done with him. Dragon Ball Super: Broly differed from past movies by making Broly into a sympathetic character. Though Broly was the main villain, it was made clear that he isn't a bad person. While in his base form, he actually has the qualities of a peaceful, innocent man.
If Broly were to learn the same ability that Kale has in Dragon Ball Super, he could fight alongside Goku and Vegeta, rather than against them. As things currently stand, Broly is stronger than both of them, so there's no mistaking that he'd make a powerful addition to the Z-Warriors. The problem though, is that with all his power, he'd outclass the two main protagonists. Having Broly around could encourage Goku and Vegeta to train even harder.