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Tao Feng: Fist of the Lotus
Studio Gigante Microsoft
Boss in 1P mode, normally playable in 2P mode.
Xbox 3/18/2003
injury
undulation
Fighting Games Today
Marvel vs. Capcom Origins
Xbox Live Arcade
9/26/2012
Compilation release
Marvel vs. Capcom Origins
Xbox Live Arcade
9/26/2012
Compilation release

While Marvel vs. Capcom includes an Orange Hulk, the comics did not have one until Uncanny X-Force #12 in 2011. A native of Earth-295 (Age of Apocalypse), he is empowered by solar radiation instead of gamma. It was never disclosed who this version of Orange Hulk was, as Banner also appears in this universe without any difference in his Hulk colors.
Marvel vs Capcom
Hulk
An almost completely finished version of Biollante exists hidden in the game code, only really missing a Rage move (and on the Xbox, her life bar portrait). Game will be unstable if attempting to hack her in, unfortunately.
Godzilla: Save the Earth
Biollante
Dark Prison
In MK Deception's internal files, the Reptile that appears in the Dark Prison stage is labeled as "Khameleon" - with a K, which is reserved for the female kombatant. The developer who wrote the name may have mistaken the two and it was originally supposed to be Chameleon, or perhaps it was indeed supposed to be the female Khameleon at one time, but either way, it's still Reptile in-game.
Mortal Kombat: Deception
Mortal Kombat: Unchained
Chameleon
Khameleon
Reptile
Sonic Championship is reportedly the result of AM2 designer Masahiro Sugiyama slipping Sonic and Tails into Fighting Vipers out of boredom. Sega big cheese Yu Suzuki happened to see this, and liked the idea, presenting it to Sonic creator Yuji Naka. Naka approved, and instead of just including Sonic and Tails as secret characters, AM2 decided to build a completely new game around the characters. Sonic can still be accessed in the arcade version of Fighting Vipers through extensive hacking.

Image courtesy of The Cutting Room Floor (tcrf.net)
Fighting Vipers
Sonic the Fighters
Sonic
Tails
Rather substantial evidence exists that Ash Williams from the Evil Dead/Army of Darkness franchise was supposed to be a DLC character for MK11. First, his name and some dialogue was sussed out when the Switch version of the game was datamined, which also included references to other characters that were actually released. Then, a teaser trailer for upcoming DLC had a chainsaw sound and a copyright notice. After that, a WB Games marketing email stated he would be arriving. All the while, Ash's usual actor Bruce Campbell flip-flopped (potentially unintentionally) on whether he was voicing in the title. Raiden/Joker's voice actor Richard Epcar once stated in an interview that Ash was going to be in, although he may have just heard the statements based on the evidence and assumed them true. Ultimately, Ash didn't make it in, and Ed Boon hasn't stated definitively what happened, but licensing rights is the best guess most have made.
Mortal Kombat 11
In the European release of the Super NES version of World Heroes, Brocken's sprite is edited to remove all references to his affiliation/resemblance to the Nazi party, presumably to appease German laws outlawing depiction or iconography of them in art.
World Heroes
Unknown
Far and off screen on Gill's stage is a sprite of Doraemon from the manga/anime of the same name for some reason. Normally he can't be seen unless a glitch is used to break the boundaries of the stage.

Image courtesy of VGFacts, but I think they got it from someone/where else in the first place.
Street Fighter 3: Third Strike
In 2017, Eternal Champions executive producer and lead designer Michael Latham pitched an idea for "Eternal Successors", a spiritual sequel to Champions, with the possibility of "upgrading" it to a full sequel if he could buy/license the IP from Sega. With Latham's passing in 2021, these plans probably aren't going anywhere.
Eternal Champions
Eternal Champions (CD)
Marvel's New York (Night)
She-Hulk stands on the right side of the street in formal wear.
Marvel Tōkon: Fighting Souls
She-Hulk