When it it was revealed the first post-series Dragon Ball Super film would feature Broly, many fans had concerns. Broly appeared prior in three Dragon Ball Z films and proved hugely popular in his first film, Broly: The Legendary Super Saiyan. That popularity led to two follow-up films, which were less well-received. Many fans grew bored of how Broly became less nuanced in each film. Broly was never an interesting character, but by the time he returns as a bio-genetic clone who dissolves in water, the Legendary Super Saiyan had become a joke. Yet Dragon Ball Super: Broly proved Broly could be a fascinating, compelling antagonist despite those earlier mistakes.
This success then begged the question: what other non-canon villains could be reincorporated into Dragon Ball Super in an effective, interesting way? Even more so, which villains would work?
Cooler
Of all the non-canon villains who could return, the one with the biggest chance of making a comeback is Cooler.
Cooler is one of the most beloved Dragon Ball Z movie villains. He's appeared in a ton of video games and his design is unforgettable. He is the only Dragon Ball Z movie villain, other than Broly, to have a second film. He fights Goku not out of a particular grudge toward Goku, but rather because of his rivalry with Frieza. This rivalry allows Cooler to let baby Goku escape from Planet Vegeta to spite Frieza. He fights Goku to beat the man who defeated his brother. This motivation is so petty and fascinating.
The main issue with a lot of memorable non-canon villains in Dragon Ball is that they appeared in earlier periods of Dragon Ball -- periods where the heroes were far less powerful. Cooler stands toe-to-toe with Super Saiyan Goku in the immediate aftermath of the Frieza Saga and almost kills the hero before the Cell Saga. This same Cooler would now be beaten in one-punch by Super Saiyan Blue Goku.
However, this problem no longer exists if Cooler were introduced in Dragon Ball Super. One thing we learn about Frieza in Dragon Ball Super is that, if he were to train for even a little while, he'd match Goku in terms of raw power. What if Frieza's brother traveled far, far away, trained, and returned home to be on-par with modern Frieza? That raw amount of power puts him on par with Goku.
Of all the villains Dragon Ball Super could incorporate, Cooler would be the easiest. He is part of a family we are already familiar with.
Bojack
Bojack Unbound is one of the more well-liked Dragon Ball Z movies, though this has very little to do with its titular villain. This is a shame, since Bojack and his crew have the potential to be fascinating villains. Bojack and his pirate crew are sealed away within a star by the four Kais. When King Kai's planet was destroyed by Cell, that seal broke, allowing Bojack to escape.
Bojack's crew can create a network of interesting villains, all of whom are underdeveloped in the film proper, but, given more time, might become more interesting antagonists. Looking at the pirate team's dynamic as well as how and why they came to join Bojack could make for an interesting story.
Bojack was sealed away potentially thousands of years prior. What makes this potentially more interesting is how Bojack might have trouble acclimating to the new era and the changes that have taken place. This could potentially result in more Dragon Ball history being revealed, particularly about old societies that have risen and fallen. Bojack can provide a real interesting insight into Dragon Ball's past, and it would be interesting to have him return.
Lord Slug
Lord Slug isn't a fan favorite villain at all, and that's honestly a shame. Namek is a big part of Dragon Ball lore, but ever since the Cell Saga, the planet has proven increasingly less relevant. Sure, the Namekian Dragon Balls helped save the day in the Buu Saga, but they've barely been mentioned in Dragon Ball Super.
Lord Slug, in the original film, was one of the Super Namekians -- a tribe banished for breaking tradition. In the film, this is due to them gaining the ability to grow super large (a technique Piccolo himself used in Dragon Ball). However, what if that break from tradition was more severe and tied into the very lore of the Dragon Balls themselves?
There are a lot of ways Lord Slug can be re-imagined. The first time Lord Slug appeared, he didn't get enough time to shine. Perhaps this time he can evolve into something truly interesting.
The Tuffles
Tuffles are a race that lived on Planet Vegeta before the Saiyans took over. This ancient race has appeared multiple times across Dragon Ball history, yet never did their full potential full appear. These are a disenfranchised people, stuck without a world thanks to the actions of the Saiyan race. The sins of the fathers will be paid onto the sons, with the descendants of the Tuffles taking vengeance on Goku and the others -- perhaps thanks to a little genetic modification.
Tuffles, it must be said, did appear during Dragon Ball Super's Tournament of Power, but perhaps Dragon Ball Super can return to a few prior non-canon Dragon Ball Tuffle villains in order to maximize their potential.
Previous Tuffle antagonists included Dr. Lychee and Hatchiyack from The Plan to Eradicate the Saiyans and Baby from Dragon Ball GT. Both have the potential to be truly dangerous antagonists, especially Baby, whose body swapping abilities can prove absolutely devastating in the open-multiverse of Dragon Ball Super. What if Baby overtook the body of Beerus -- or worse, Zeno?
The Shadow Dragons
Arguably the greatest concept Dragon Ball GT ever presented was that of the Shadow Dragons. What better antagonist to serve as Dragon Ball's final antagonist than the very Balls they've relied on all this time? With each wish, a dosage of negative energy fills the balls. Due to Goku and friends' overuse throughout the series, the Dragon Balls have overloaded. The subsequent meltdown results in each Dragon Ball becoming a sentient Dragon.
The concept's huge potential was mostly squandered in Dragon Ball GT, but in Super it could potentially serve as something truly special: the heroes forced to confront the sins of their past in an epic battle. Even more interesting, it could prove to dive even deeper into Dragon Ball lore, considering now we know about the existence of the Super Dragon Balls.
Regardless, of all the Shadow Dragons, Omega Shenron remains one of the most memorable and visually spectacular villains in all of Dragon Ball GT. To see him return, this time on par with the literal Super Saiyan Gods, would be a treat to the eyes.