Dragon Ball's output can be split into canon and non-canon material. Although those are often flexible (especially with Akira Toriyama at the helm), "canon" generally describes material released in manga form or written with the original creator's express blessing and involvement. "Non-canon," on the other hand, usually refers to anime-only filler episodes and movies that aren't included in the prime continuity.
Until recently, Dragon Ball movies were entirely non-canon, but Battle of Gods bucked that trend, kick-starting a string of official Dragon Ball movies written by Akira Toriyama, the most recent being Dragon Ball Super: Broly in 2018. The Broly character first appeared in 1993's non-canon Dragon Ball Z: Broly - The Legendary Super Saiyan, still bearing a grudge against Goku for his incessant wailing as an infant. In a first for the franchise, the 2018 movie formally enshrined Broly into canon. Dragon Ball Z: Broly was a critical and commercial success and fans responded positively to witnessing Broly's modern reinvention, especially with the "crying baby" backstory nowhere to be seen.
By bringing Broly into the Dragon Ball fold proper, Toriyama set a precedent that could be exploited for future movies. The Dragon Ball story is peppered with unofficial characters who, while perhaps not as popular as Broly, have something to offer the modern era. Here are the other characters Dragon Ball should canonize in future movies.
Pikkon
One of Dragon Ball's coolest creations regardless of canon status, Pikkon is a character many don't realize is filler while watching for the first time. After dying in the battle against Cell, Goku begins his time in the Other World, and is delighted to learn there's a martial arts tournament in town. The final match comes down to Goku and a green-skinned fighter under West Kai's tutelage called Pikkon. Although the character is a shameless Piccolo ripoff both in of design and personality, Pikkon fights evenly with Goku's Super Saiyan 2 form and boasts innovative ki attacks that separate him from the existing Z-Fighters. Given how drastically Piccolo falls behind Goku in Dragon Ball Z, Pikkon is a breath of fresh air - a glimpse of what the Namekian might've been.
With a few personality tweaks to make him less Piccolo-like, Pikkon could become an exciting ally to Goku in a future movie or anime series. If Dragon Ball Super ever holds another inter-universe tournament, Beerus and Whis might introduce Pikkon as the first Universe 7 team member not from Earth (or called Monaka). Pikkon enjoys a level of appeal that filler characters rarely attain, and subsequently appeared in the non-canon Fusion Reborn movie, suggesting a future transition into main continuity would be well-received.
Garlic Jr.
Against the likes of Frieza and Cell, Garlic Jr. is a relatively pathetic Dragon Ball villain. With a Pilaf-esque appearance and henchman who collectively refer to themselves as "the Spice Boys," Garlic Jr. is hardly the most intimidating antagonist, but his appearances in anime filler and The Dead Zone add fresh ingredients to the Dragon Ball stew. On his two separate attempts for world domination, Garlic Jr. is defeated by Gohan, giving Goku a well-deserved break from the limelight. With Dragon Ball Super focusing squarely on Goku and Vegeta, the canon debut of Garlic Jr. could showcase the lesser Z-Fighters. Furthermore, Garlic Jr. was the first villain to successfully ask the Dragon Balls for immortality, marking this pungent villain out as a very different kind of threat.
Garlic Jr. isn't important enough to sustain an entire movie, nor would he be likely to trouble Goku or Vegeta in their current forms. But Garlic Jr. would prove a worthy addition in a future Dragon Ball Super arc, giving under-used ing characters such as Gohan, Tien, Goten and Trunks more screen time.
Tapion
With his bright red hair and broadsword, Tapion is arguably the most striking unofficial character in the entire catalog of Dragon Ball movies. Appearing in 1995's Wrath of the Dragon, Tapion is an alien hero who carries an ocarina and harbors a legendary, raging monster in his body. Tapion looks every inch the classic anime protagonist, and bears thematic similarities to Naruto. Because Tapion carries a monster in his soul, the morally black-and-white world of Dragon Ball would gain a rarely conflicted good guy.
As he endears himself to the Z-Fighters, Tapion's tortured past and heroic nature come to the fore, and the flame-haired character is a natural fit standing shoulder to shoulder with Goku, Vegeta and the others. Wrath of the Dragon also offered one telling of how Trunks obtained his sword. While it's easy to draft new villains into Dragon Ball canon, allies are just as important. Tapion ranks alongside Pikkon as a popular filler character who would compliment the current Z-Fighter line-up but, unlike Pikkon, Tapion doesn't copy an existing character, so wouldn't require a makeover before debuting in Dragon Ball Super.
Android 13
Like Garlic Jr., Android 13 would add variety to any future Dragon Ball Super anime series. In the canon continuity, most of Dr. Gero's androids remain unseen, paving the way for an appearance from the evil 13. Pitting Ultra Instinct Goku against a robot would result in a very quick battle, but Android 13 could play the antagonist role in an anime arc starring Androids 17 & 18. The Tournament of Power reintroduced Android 17 to Dragon Ball, and he's since become a fully-fledged member of the Z-Fighters. Now the android twins are reunited, their development could continue with a special one-off story, where 17 & 18 set out to destroy the remnants of Dr. Gero's experiments and are forced to battle their most deadly predecessor, Android 13. Not only would an android vs. android battle offer a compelling clash, the arrival of Android 13 could force 17 & 18 to confront the traumatic past events that led to their transformations.
Omega Shenron
Although Dragon Ball GT is widely regarded a weak period in Dragon Ball's origins and pay-off Old Kai's warning that the Z-Fighters were too reliant on the mystic orbs.
Goku Jr. & Vegeta Jr.
Another of Dragon Ball GT's strengths lies in its ending, which was arguably better than Dragon Ball Z's canon finale. Where Goku's story originally ended with the Saiyan embarking on his next adventure training Uub, the Dragon Ball GT conclusion dealt with Goku's legacy, flashing forward to a future where the descendants of Goku and Vegeta compete in a martial arts tournament. When the Dragon Ball Super era comes to an end, revisiting Goku Jr. and Vegeta Jr. may prove a worthwhile endeavor, especially with the franchise now firmly orbiting those two characters. Exploring these young Saiyans would afford Toriyama a final opportunity to reflect on Goku's long journey, while potentially setting up a spin-off based around the next generation of warriors in Dragon Ball.