Cooler should be the villain of the next installment in the Dragon Ball franchise, whether it be in a sequel to Dragon Ball Super: Broly or another anime series. As the most popular non-canon villain in the world of Dragon Ball, it's hard to imagine a better choice for Goku's next major antagonist.
After 133 episodes, Dragon Ball Super came to an end with Universe 7's victory in the Tournament of Power. The series was followed up by the 2018 movie, Dragon Ball Super: Broly, the first film to carry the "Dragon Ball Super" name. The story of the film saw Frieza hatch a scheme that pit the long-lost Legendary Super Saiyan, Broly, against Goku and Vegeta. Broly's power forced Goku and Vegeta to become Super Saiyan Blue Gogeta for the first time ever. The movie, which grossed $124.5 million at the worldwide box office, became a huge success for the franchise.
Dragon Ball Super is over, but the anime is expected to come back in some form - though it's not clear how or when. Some comments have suggested that a second Dragon Ball Super movie is the next step for the franchise, but this hasn't been confirmed. Either way, both Dragon Ball Super and Broly were easily successful enough to warrant a follow-up. However, Broly doesn't set up the next installment, so where Dragon Ball goes next is open to speculation. Here's why the next Dragon Ball anime or movie should use Cooler as its villain.
Cooler Doesn't Exist In Dragon Ball Canon (Yet)
Cooler has appeared in two non-canon Dragon Ball Z movies, making him one of only two characters to be the villain in more than one Dragon Ball Z movie (the other being Broly). Cooler was introduced in 1991's Dragon Ball Z: Cooler's Revenge, which was set after the Frieza Saga. At the time, the strongest opponent Goku and the Z-Warriors had ever faced was Frieza, but the movie revealed that Frieza's older brother was even stronger than him. After hearing of Frieza's death, Cooler travels to Earth and battles Goku, Piccolo, Krillin, and Gohan in an effort to reclaim his family's stolen honor. Cooler achieves his final form - which is unique to Frieza's version of the ability - and though he pushes Goku to his limits, Goku goes Super Saiyan and manages to defeat Cooler. Cooler reappears in one more movie, Dragon Ball Z: The Return of Cooler.
Since all Dragon Ball Z movies aren't canon, none of the new characters in the movies ever actually met Goku and the others. That being said, that doesn't mean those characters can't exist in Dragon Ball. Though Broly had already been the villain in three movies, Dragon Ball Super was able to reintroduce him, since its movie is considered canon. So while it's true that Cooler has never been seen in the series, he can still be utilized in the future. Also, it's worth pointing out that now that Broly (and Gogeta) are official included in the show's continuity, Cooler is now the most well-known Dragon Ball character not in canon.
How Dragon Ball Super Can Use Cooler Differently
Since Broly previously existed outside of continuity, Dragon Ball Super was allowed an opportunity to provide a fresh new take on the Legendary Super Saiyan, which gave him a much deeper backstory. Dragon Ball Super can do something similar with Cooler. Like Broly, a movie or show with Cooler doesn't have to rehash the same story. In fact, the current state of things in Dragon Ball Super makes using the same Cooler story an impossibility.
The whole premise behind Cooler's Revenge wouldn't work in Super. The movie was about avenging Frieza's death, but thanks to Goku's decision to wish back Frieza with the Dragon Balls, revenge is no longer necessary. With Frieza back among the living, Cooler would need a new reason to fight Goku.
Also, Dragon Ball can create a story that uses both Frieza and Cooler, which opens up numerous possibilities for the story. Frieza could seek to use Cooler against Goku in the same way that he did Broly, or he could be forced to swallow his pride and ask Goku to fight Cooler for him. Regardless of what direction they take, it won't be anything like Cooler's Revenge or The Return of Cooler.
Cooler Can Be Dragon Ball's Strongest Villain
At this point, Cooler's strength wouldn't be able to compare with Goku's, so the new canon-version of Cooler would need his power scaled so that he could match up well with Goku and Vegeta. Dragon Ball Super's Golden Frieza Saga explains how this can work. Apparently, Frieza's species is so naturally powerful, that Frieza only needed to train a for a few months to get back on Goku's level. This means that Frieza can catch up to Goku anytime he wants. Just four months of training does more for Frieza than it ever could for any of the characters -- except, of course, for Cooler. If the same rule applies to Frieza's brother, it makes sense that if there's any unrevealed character in Universe 7 who can pose a threat to Goku, it would be Frieza's older brother. As strong is Frieza is, Cooler could be even stronger.
Cooler being more powerful than Frieza feels right for the character, since that was how he was introduced in the first place. If Cooler is more dedicated to his training that Frieza, he could be levels above all other enemies Goku has fought in the past. What Frieza is capable of accomplishing with just four months of training makes one wonder what a superior member of his species could do with an even longer training period.
Furthermore, the anime can take advantage of Frieza's Dragon Ball Super upgrade, which has been dubbed his "golden form". As Golden Frieza, he forced Goku to go Super Saiyan Blue to beat him. The golden form was given to Cooler in the web series, Dragon Ball Heroes (which is also not canon). Dragon Ball Heroes made the villain more dangerous than ever, so Golden Cooler is certainly something that might happen if the character gets the movie treatment again.