So far, the character roster for Street Fighter 6 is shaping up to have a healthy mix of newcomers and old mainstays. Not everyone's favorite fighter has managed to make the cut, but the inevitable future DLC means plenty of classic characters still have a chance to make it.
But then there are characters in the Street Fighter universe whose chances of appearing are significantly lower, for one reason or another. For some, it wouldn't make narrative sense for them to return; for others, they may be part of an already well-represented character style or archetype. And then there are those who simply don't have the support behind them to make their addition worth the trouble.
6 Necalli
One of the four new characters introduced in Street Fighter 5, the soul-sucking Aztec with the wild hair Necalli is an example of failing to live up to hype. He seemed poised to become a major antagonistic force within Street Fighter lore, but ended up playing barely any role at all. While Bison remained the main villain, Necalli spent most of his screen-time as a minor nuisance, suddenly popping up only to get swatted away just as quickly.
Necalli's biggest story contribution was being the catalyst for awakening Ryu's Power of Nothingness. He was last seen being obliterated by Akuma after failing to devour his body and soul, going out without a single decisive victory to his name. With his mystique stripped away, few are clamoring for Capcom to give him another shot.
5 Guy
Guy made his Street Fighter debut back in the first Street Fighter Alpha, alongside fellow Final Fight character Sodom. Many more characters from Capcom's iconic beat 'em up, such as Cody, Rolento and Poison, would migrate in their wake. Guy's last playable appearance was in Ultra Street Fighter 4, which had the largest concentration of Final Fight characters in the series to that point.
Since then, the Bushinryu practitioner has yet to appear on any more recent Street Fighter rosters. His master Zeku usurped him for Street Fighter 5's season 2 DLC, and it seems as though he will similarly be replaced in Street Fighter 6 by his new pupil, Kimberly. She appears to share a few of his moves, albeit with some extra twists of her own, so having both characters on the same roster would perhaps be superfluous.
4 Abigail
When this former Mad Gear Gang thug was revealed as a DLC fighter in Street Fighter 5, the backlash was practically immediate. Fans were immediately turned off by his absurd musculature, childish personality, and the fact that more popular Final Fight characters hadn't made the cut instead.
Some players eventually warmed to him once they got the chance to play him, but few characters in Street Fighter ever had such a negative initial reaction as Abigail did. Capcom appears to be positioning Street Fighter 6 as a course correction, avoiding many of the pitfalls that 5 ran into. It would make sense for them to avoid reintroducing such a hugely controversial character.
3 Evil Ryu/Kage
Originally appearing in Street Fighter Alpha 2 as a hidden character, Evil Ryu is what becomes of Ryu when he gives into the power of the Satsui no Hado. He was eventually able to overcome its temptation and instead embraced the Power of Nothingness, but shedding his evil side created an entirely new entity known as Kage, a physical representation of Ryu's potential for evil.
Kage received a mixed reception from fans, primarily due to his awkward appearance and not being as strong as previous incarnations of Evil Ryu. Kage's story mode in Street Fighter 5 concludes with him vanishing before Ryu, as he rejects the Satsui no Hado for what appears to be the last time. With this character arc of Ryu's over, there appears to be little reason for Evil Ryu or his derivatives to make his return.
2 Gill
Gill was the main antagonist of Street Fighter 3, the leader of the mysterious Secret Society. Unlike Bison and his Shadaloo organization, who committed evil deeds for his own benefit, the Secret Society performed their morally questionable acts towards a perceived greater good. Despite this added ambiguity, Gill was never as popular with the fanbase as his more malicious predecessor. He was MIA throughout the games until finally reappearing in Street Fighter 5, set before the events of 3.
It's possible that Gill, or some other vestige of the Secret Society like Urien or Kolin will return in Street Fighter 6, if only to give some closure to Street Fighter 3's story, but that may not be Capcom's priority right now. At this time, it is unknown who the main villains of Street Fighter 6 will be (although there may be some hints). It wouldn't be surprising if Capcom would prefer to focus on building up the new antagonists, rather than surrounding them with the foes of old.
1 Charlie Nash
Guile's best friend who was slain by Bison, Charlie became a playable character in the prequel series, Street Fighter Alpha. Since then, his appearances have been sporadic, typically only showing up in spin-off and crossover games. It wasn't until Street Fighter 5 that he made his return, literally coming back from the dead as a brooding, Frankenstein-esque creation of the Secret Society, with a new set of moves to separate him from Guile's charge character archetype.
At the end of Street Fighter 5's story mode, Nash sacrificed himself in a vain attempt to kill Bison via self-destruction. It's still possible that he could return in a later sequel, especially considering he's returned from the dead once before, but for now it may be best for Charlie to have another much-needed rest.