A sign of an exceptional fighting game is the diversity in characters across its entire lineup of fighters. A team fighter like King of Fighters or Dragon Ball FighterZ thrives on team composition, capable of providing both deep character variance, or similar archetypes dedicated to a specific gameplan. A strictly one-on-one fighter succeeds both in providing the character archetypes players expect (rushdown, zoners, etc.), alongside new characters that bend the rules to provide a more unique fighting style. Street Fighter has almost always been a series that's largely excelled with character diversity, and with Street Fighter 6 on the way, many are cautiously excited for more.
As with any new Street Fighter entry, the inevitable question becomes which characters are returning? Theoretically, the timeline of the overarching Street Fighter story can hint at who to expect will rejoin the roster, but that's never stopped Capcom from adding unexpected characters before. Street Fighter 5 is a prime example of this, with a half-dead Nash and a prisoner-turned-Metro City mayor version of Cody returning with the fifth entry. There's a degree of unpredictability with Street Fighter 6 that allows for potential choices for returning characters to span the wealth of iconic Street Fighter characters throughout the series' history.
Fei Long
One of the most iconic Street Fighter characters since his inclusion in Street Fighter 2, Fei Long is one of many fighting game tributes to the iconic martial arts film star Bruce Lee. Arguably one of the closest representations to Bruce Lee's depiction in film, Fei Long looks and plays like the famous Hong Kong martial artist that's arguably responsible for fighting games today.
Fei Long was sorely missed in Street Fighter 5 (outside of a cameo on the Bustling Side Street stage) and deserves a return to the ring. His fighting style is unique among other fighters for his poke-and-prod neutral game and high-damage punish game. Sure, plenty of other characters are high-damage rushdown fighters, but Fei Long's reverence for Bruce Lee's iconic Kung fu is particularly unique. His Lee-esque vocal and fast-paced fighting style is unlike any other rushdown character in the series.
Adon
Serving as Sagat's apprentice for a short time, one of the few strictly Muay Thai-focused fighters in Street Fighter's history is Adon. He's a boisterous and arrogant fighter who seeks to become the god of Muay Thai martial arts, especially after Sagat's defeat at the hands of Ryu. Unlike Sagat, who is an often-grounded zoner with excellent fireball and anti-air coverage, Adon prefers aggressive pressure via kicks and airborne strikes. Adon's a vocal and acrobatic fighter who works as an antithesis to shoto fighters, and deserves a return alongside his forsaken master as well. His high-flying and pressuring style isn't like too many other fighters in Street Fighter.
Gen
While Zeku to some extent served as the "old master stance character" archetype in Street Fighter 5, Gen used to be the prime example of that same type of fighter. Connected to characters ranging from Chun-Li to Akuma, Gen is a philosophical and calculating former assassin and expert martial artist. Last seen in Ultra Street Fighter 4, Gen is a stance character swapping between standard motions and charge motions in mantis and crane stance, respectively. He's arguably one of the more complex fighters based on his skill ceiling alone, but if Street Fighter 6 wishes to dive more into the complex character designs, Gen would be the prime choice for a return.
Akira
One character who actually did end up in Street Fighter 5, but didn't receive nearly the time or the spotlight, was Akira. Originally from the Rival Schools fighting games, she is a confident and commanding fighter, but wears her biker outfit to offset her normally friendly and positive personality. Akira was an enticing newcomer who came in way too late in Street Fighter 5's lifespan, and deserves another shot with Street Fighter 6. Her playstyle is almost similar to Karin's, focused on elbow tackles while adding a flair of air combos that's unique compared to Street Fighter's largely grounded combo game. A unique play style like that should return in Street Fighter 6 as well.
Dudley
One of the more iconic boxers in the Street Fighter, Dudley's demeanor almost serves as an antithesis to Balrog, similar to Adon and Sagat. Though the two don't exactly have an intrinsic relationship, Dudley is more of a gentleman than the bombastic Balrog. He's all about fair fights, chivalry, and respect; to some extent similar to Ryu in that regard. Dudley's fighting style focuses on clean punches and techniques, all about displaying a masterful finesse in fighting style as a suave gentleman boxer. All of the boxer-type fighters in Street Fighter 5 are all cocky and confident (Balrog, Ed, Luke to some extent), but none match the personality of Dudley.
Street Fighter 6 is in development.