Gouken exists entirely because of fans clamoring for years for Capcom to make a real "Sheng Long."
Guile's dead combat buddy from the Street Fighter Alpha games, known as Charlie internationally and Nash in Japan, has been addressed by fans under the combination full name "Charlie Nash". Thanks to his dogtags seen in Guile's intro video in IV, that full name is now confirmed as canon. It was probably done out of convenience, to avoid having to draw two different versions of the same scene with a different name on the dog tag, and UDON's comics used the "Nash" surname prior to Capcom making it official (as did The Legend of Chun-Li film around the same time), but still. Bottom line: Street Fighterfandom, you're welcome.
Michael T. Coleman voices Cody, giving him a neutral-sounding accent, dipping occasionally into a more Brooklyn-sounding accent, as evidenced with his intro and some of his attacks (i.e. "Criminal Uppa!").
Shrug of God: The Arcade Edition and the game's staff are insisting in every way possible the ambiguity of Oni's identity, he who basically all fandom has accepted as a new Super Mode for Akuma, but the in game material still says otherwise: the character is just named Oni, all of his available quotes so far bears almost no connection to Akuma aside from their power-hungry nature, his fighting stance resembles Gouken's more than anyone else, his face has minor differences compared to Akuma's (notably the nose), and the already leaked ending is even more dull than Akuma's as one cannot tell anything about the character by just watching it (well, except the fact he can destroy volcanos with a single punch and possess flight abilities); still it is safe to say that the logical conclusion is that Oni indeed is Akuma. Word of God finally confirmed Oni as Akuma via Seth Killian.
Trolling Creator: Yoshinori Ono openly admits to being this. For example, Decapre's Introduction video quickly gets the fandom's hopes up and swiftly dashes them, with R. Mika standing up to Bison and vowing to defeat him, only for her to be easily blasted away.
Originally, Rufus was named "Cobra King," and was a lean muscular black man wearing a stylish karate gi◊. He was still planned to be acrobatic fighter, but employed a fusion of breakdancing and kung-fu (breakung-fu) instead of his self-taught kung-fu. Marketing research on the preferences of American gamers had a big hand in Cobra King's design.
Then, SF4's art director suggested that Cobra King should be made overweight. The series of changes after that led to Cobra King becoming Rufus. According to SF4's producer, they used Rufus' current look because it would "freak people out," and because "over-the-top" characters sets Street Fighter apart from the other fighting game franchises.
The gi would later be used as an alternate outfit for Ken.
T. Hawk and Dee Jay were originally going to be in vanilla IV. Both of them joined the roster with the release of Super.
R. Mika, Karin, and Elena were considered for Super. Series Producer Yoshinori Ono said that if DLC characters are released, R. Mika will be among the first. In fact, Elena barely missed the final roster for Super Street Fighter IV.
Ono also mentioned that he wanted Final Fight's Rolento in the mix as well.
And now with Ultra Street Fighter IV coming along, Elena and Rolento are finally playable characters.
Concept art shows that Guy's alternate outfit in IV was planned to be Strider Hiryu.
As detailed under Never Trust a Trailer on the main page, more characters would have received a second Rival as opposed to just six (Ryu, Chun-Li, Guile, Cammy, C. Viper, Seth):
E. Honda: Adon
Balrog (Boxer): Cody
Vega (Claw): Cammy
M. Bison (Dictator): Cammy
El Fuerte: Guy
Fei Long: Gen
Rose: M. Bison
Guy: M. Bison
Additionally, Cody's Rival was originally slated to be Chun-Li, as seen in the SSFIV trailer. Considering that Cody's an escaped convict/vigilante and Chun-Li is an Interpol agent, as well as factoring in their respective win quotes for each other, it makes sense; later in development, Cody's Rival was changed to Guy, his old Final Fight brother-in-arms and SFA3 sub-boss (which also makes sense, albeit for more personal reasons as opposed to merely being on opposite sides of the law). The trailer also featured locales different than the final build for all of the new characters' Rival Battles.