We know that the 8 World Warriors from SF2 are making a Comeback on 6, but not the bosses or 2 of the Super SF2 folks. I'm going to look at characters from different SF Games and try to be objective on reasons for them to return and reasons for them to not return. Then I will use my personal biases... for the Moment I'm sticking to 1, Alpha 3, Super 2, and 3rd Strike... mostly because I didn't play much of SF4, and I have to get reacquainted with SFV... (once I get my PS4 fixed...)
This is the first game where Ryu and Ken were the only playable characters. So I will be talking about the other guys.
Capcom technically owes the fans a playable Retsu since he was scrapped from Capcom Fighting Evolution. A Japanese Martial Artist without any ki attacks is something uncommon in SF...
It would also be interesting from a story point of view having Ryu meet Retsu after the latter was convinced by Ryu's skill to not use lethal techniques.
Personally, I like the idea of Retsu returning, but not necessarily in a playable role. Then agaim, Dhalsim still is playable, when most his contributions to the story would be the same if he was a NPC.
We know the original Geki is dead and there is a Geki II. So canonically speqking Geki can't make an appearance, but Geki II can fill in that role. While both Vega and Ibuki can make Geki "feel redundant", both of them aren't in the game, so, he could fill in their gaps. Canonically, speaking Geki II could be hunting his Predecessor's killer.
Personally, a Classic Looking Ninja is something that I won't say no to.
Another SF character that has been excluded, from the franchise. Some might argue that Cody is a Spiritual Successor to Joe, thus making Joe Redundant. Storywise he has nowhere to go.
Personally, I don't care much about him. But I can't see how he could come back without reinventing his whole character.
We now know that Mike and Balrog are 2 different people. Since Balrog is more "Tyson-like" maybe Mike could be more like a different boxer (not necessarily Ali, but it could work) The whole, teying to get money from the tournament to help his community ia a reasonable story angle. Adding him could lead to a potential return of Balrog.
Personally, I don't think he's NEEDED, but it would be a curious addition depending on how Capcom treats him.
After SFV, going from Shadaloo lackey to Knazuki butt-monkey, Birdie lost all of his edge storywise. Gameplay is the only reason to want him, but I'm not that into the online rankings for each character in competitive usefulness, so I can't form an opinion on whether he's needed or not.
Personally, I don't see a point for his return after SFV.
His Bojutsu skills could bring some flair on SF6. Storywise is the only real thing impeding his return.
Personally, if Eagle and Dudley made it to the game, I'd like to see a friendly rivalry between them. Maybe Dudley lamenting that Eagle relies far too much on sticks instead of using his fists, which is more civilized.
Since Jaime took the Drunken Fist gimmick, Lee loses a little bit of interesting features, not to mention that his nephews already took elements from him (SF3's Yun and Yang) storywise he has nowhere to go, since he retired as a fighter and is into real estate now.
Personally, I would like to see Lee return and maybe have a sort of interest in Jaime for his prowess in Drunken Fist... at least from a NPC perspective. If he's playable, cool beans! But I'm not DYING to play as Lee.
The only issue blocking Gen's return is his storyline. He's believed to be dead.
As much as I like, Gen, if he's DEAD, he shouldn't return... also, the dying by possible Leukemia has been muljed way too much. The interchangeable styles was cool, but maybe that gimmick could return with a new character.
His gameplay could be the only real argument to use for his return, because his storyline already reached a satisfactory conclusion.
Personally, I'm not a fan of Adon. I'm glad that he's a dead end storywise, but if he somehow returns, it wouldn't piss me off.
Storywise he's done. How many times can we have Sagat looking for a rematch with Ryu...
Personally, I like Sagat, but right now he could show up as a NPC, because his story potential is limited at the moment.
Super Street Fighter II:
The Lee family has no issue with Fei Long being a tribute to Bruce Lee. His gameplay, while traditional, could use some Spicing up... The only thing holding him back is his storyline.
Personally, I wouldn't mind his return, but he needs a new angle... the Bruce Lee "Clone" angle isn't that interesting anymore.
The stereotypical Native American character. Unless he goes through a big redesign, this could be considered offensive. Unless Juli of the Dolls is still a Doll, then there's no reason for him to return.
Personally, he shouldn't come back... it's not a dumb reason like "he's too tall". It's mostly about his role on the story. He did save hos village, but not the missing girls that became Dolls. Since SF6 is after SF3 and there is no Shadaloo, then, T.Hawk has no reason to be in the game.
The disgraced member of the Shadaloo Four has no realistic reasons for returning. At best, having a feud with Mike (who currently isn't in the game) would be the only plausible reason.
Personally, it's pointless to bring him back. He's just not that exciting and his storyline leads nowhere.
If I recall correctly, Vega was banned from the SFV competitive scene due to his wall climb on his stage.
Personally, I'd like to see him back, but only if his theme doesn't change too much.
Nope! Nope! Nope! He's dead and Shadaloo is no more. There is no need for him. Alpha series, 4 and 5, made Bison overstay his welcome. The Clone body method of returning has been overplayed.
Personally. I KINDA want him to return, but at the same time I kinda don't want him because his story is already done and they kept bringing him back when he wasn't wanted or needed.
He already died once or twice. Bringing him back does a disservice to Guile's story.
Personally, he can be skipped. He's a "Better Guile" than Guile and we already have Guile.let the dead stay dead and all that.
Without Bison, there is no point for Rose to exist. Gameplaywise, she wasn't well received in SFV, so it makes sense to have her sit this one out.
Personally, I don't mind having Rose, but she needs to be slightly de-nerfed in order to be more Viable.
He's a joke character, his moveset is a joke and storywise he's a joke with a dead end story, especially after Sagat let him win. Not to mention that he's a "Shotoclone".
Personally, I like Dan. He's not a good character, is incredibly annoying and I like that. I want him in the game, but not ASAP... maybe changing his schtick a little bit, like he actually got a student who admires him as a Sensei (and the student is far more competent warrior) could have more people take an interest on Dan?
Guy, Cody, Rolento, Sodom:
The only reason to have the "Final Fight guys" return is that SF6 seems to have taken Metro City as a focal point. This also applies to the Final Fight characters from other games like Hugo, Poison, Maki, Abigail, Lucia, etc.
Personally, I'd say bring on the entire Final Fight Crew, especially Haggar, so he can finally face off against Zangief.
Another "Shotoclone" and her story after SFV left her wondering if the path of the Warrior is a righteous one. She has no need to return...
Personally, I want, nay, NEED Sakura in SF6. Blame my fanboyism, but hear me out. Ken's life is in shambles because of trying to follow the path of the Warrior. Maybe seeing Ken fall that low could spark her to leave the path of the warrior, or her doubts could make Ken want to go home and be a family man. Hell, maybe she's the one that makes Ryu settle down and become a family man... (this ship will never die!!!)
Her new role as the head of the Kanzuki Zaibatsu makes her a player to track down odd events. Her role can still work with her as a NPC.
Personally, having Karin without Sakura is like having Ken without Ryu. But she has more chances of being important to the story even in NPC form than Sakura does.
She brings nothing to the story. Gameplaywise, she's good-ish... IIRC, she's a B-Tier on competitive rankings.
Personally, I'd rather have Yamato Nadeshiko (Mika's tag partner) so we could get something SIMILAR to Mika, but at the same time a "fresh experience" from a new playable character.
Since Bison is no more, there is no real reason to bring them back. Especially if they're Cammy Clones.
Personally, I'd rather have them as NPCs tied to other character's story. (Ie, Juli and T.Hawk, Decapre and Cammy, etc.)
Street Fighter 3: 3rd Strike:
Since he's the champ at the end of SF3 it makes sense for him to return.
Personally, I don't care about him, but he still does have a bit of potential.
His potential for being part of an overarching story is limited and his role is better suited for a NPC cameo, unless Capcom brings characters that could work well with Dudley on story level.
Personally, I'm not a fan of his gameplay, but I would love to see him complementing another character's story. Whether it's Karin, Eagle, Mike, or Balrog, I don't care.
I'm not sure how would she fit in canonically. Gameplaywise, she should return, especially since we're getting some flashy styles and Capoeira would look badass.
Personally, sure! The more the merrier... and no, it's not because of Only the Strong...
It would be interesting knowing where Ibuki stands after SF3 As a character and as a Ninja.
Personally, I don't care much aboit Ibuki, but since she's a ninja, I want her in the game... especially if Geki II is added.
He hasn't appeared as a playable character outside of SF3. Maybe since SF6 is after 3, his gameplay has evolved to rise from the lower tiers he's been relegated to in SF3.
Personally, I wouldn't lose any sleep if he doesn't make it to SF6. Maybe making Effie Playable with Necro assisting in some moves like Mika does with Nadeshiko would be an interesting alternative.
Since Ryu seems to have become an "apprentice of sorts" to Oro, it would be interesting to have him on SF6 even if he's just a NPC.
Personally, I wouldn't lose any sleep if he doesn't make it... While I'm not a fan of the character, it would be cool to see him fighting the new generation of Fighters... based on his canonical role of Greatest hermit warrior.
The G-Project's perfected weapon... with Seth being a sort of Copycat and in some cases better than SFV's Eleven, since Twelve has a bit more control over the transformation... and canonically speaking Zombie Charlie was made with pieces of Eleven.
Personally, I don't care about Twelve or Eleven for that matter. They may have a cool transformation gimmick but they're basically soulless puppets so I don't care about them.
His story is done and he has nothing to contribute to the game. He's also a forgettable character.
Personally, I have no idea who you're talking about? Ah! The Edgelord Iori wannabe with a hint of Hyoga the Swan... Shit, I can't believe I forgot him... then again, I also forgot C.Viper despite her 2 huge reasons for being unforgettable: Her design being another SNK wannabe and appearing in UMVC3...
Yun and Yang:
With Jaime wanting to be like then, he should meet his heroes and get his ass kicked by them, so he forges his own path.
Personally, I don't care much about them, but I mist admit that they have potential to move the story forwards... but I still wouldn't lose any sleep if they don't make it.
Like Retsu, her Martial Art is more grounded in comparison to the rest of the fighters. Her only issue is her storyline, which is a tad repetitive: My Dojo is in danger, let me bust other Dojos to prove the Superiority of mine... She could almost be Dan's new rival...
Personally, an Akane Tendo wannabe is alright with me. Capcom, can you make a Ranma½ fighting game pretty please?
He is rumored to be a NPC in this game. It would be interesting to see him fight and his evolution as a fighter.
Personally, I'd only take Sean if he can finally fight Dan (and lose against the Saikyo master).
He's an unsolved mystery that led nowhere... maybe it's time for Capcom to throw fans a bone and reveal something.
Personally, I don't care about his gameplay. The mystery is what bothers me the most... especially, since Q is a lot better than G.
If the ending of 3rd Strike is canon, then Gill lost and Urien is the new Illuminati boss. He NEEDS to be there since the Illuminati weren't completely beaten after 3rd Strike.
Personally, I could take Urien as a NPC, because he isn't THAT important in comparison to Gill.
The big boss of the Illuminati, assuming Urien's ending isn't canonical. Like I said on Urien, The Illuminati story hasn't truly ended and there is more storytelling potential here.
Personally, I HATE HATE HATE GILL! "The mark of my dignity shall scar thy DNA!!" Brings bad shivers down my spine. Unfortunately, he still has a larger role to play in the story.
I think that's all of them, not counting SF4 and 5 characters. BUT SINCE this rant took far longer than expected, here's 3 characters that I absolutely DO NOT WANT in SF6:
Since I've only played Vanilla SF4, I've encountered Broken Rufus way too many times and I fucking Hate him... The fat fuck is far cheaper than Seth. Not to mention that he was made extremely annoying on purpose... not just on gameplay, but his personality. I wouldn't want him on SF6 at all, not even as a NPC.
This is the only character in SFV that I hate almost as much as Necalli, but unlike Necalli, F.A.N.G. is nearly unplayable. (As in you'll be losing a lot by using him...) Unlike Rufus, he's tolerable as a NPC... in small doses, but still, DO NOT WANT!!
Garbage character is garbage. I'd take Rufus and F.A.N.G. slathered in Hakan's oil as they fondle the non-existing scrotum of Seth as they hum the World Warriors theme instead of Necalli. Storywise, Necalli is Garbage. Gameplaywise he is garbage as well... he apparently barely beats Blanka in performance, but ranks beneath Dan, who already ranks a couple fighters beneath F.A.N.G.! Necalli is worse than Dan. He was almost as disappointing as Captain Phasma. Fuck Necalli.