When Cyborg forced to side with Alternate Deathstroke he says "You will follow my lead." a line Aqualad had in Young Justice, and his voice briefly softens to sound like his Aqualad voice.
Banned in China: And in the United Arab Emirates. The game's name was changed to Injustice: The Almighty Among Us for the countries in the Middle East to avoid conflict due to the God name, but in the UAE they still refused to allow the game there.
George Newbern as Superman. Aside from being a Role Reprisal from Justice League, which featured the Justice Lordsnote An alternate universe version of the League who had taken a zero tolerance policy on crime following the Flash's death, and Superman vs. the Elite, which deconstructed the idea of Superman undertaking a zero tolerance policy on crime; he's also known for playing Sephiroth, another Fallen Hero who was once considered the greatest to have lived.
Kimberly Brooks as Batgirl, having previously played Oracle, the paralyzed Barbara Gordon, in the Batman Arkham Series. To some level, this is also an Actor Allusion, as Barbara's backstory in the game states that she was originally Oracle before becoming Batgirl.
Flip Flop of God: After hinting on Twitter that certain characters might join the roster, Ed Boon had mixed opinions on the inclusion of Aquaman. A portion of his feed is alternating between "of course" and "no way" before finishing with this statement.Then he was confirmed as a character.
The Killing Joke: The Joker sets out to drive one of Batman's allies to despair by targeting their family. Only this time hesucceeds.
The Dark Knight: The Joker murders the lover of Batman's best friend and then gives said friend a Hannibal Lecture that drives him to corruption.
Kingdom Come: The Joker causes the deaths of several people Superman loves, including Lois Lane, and Superman himself loses touch with humanity. The Joker's actions also lead to his own death.
"Brave New Metropolis": Superman's descent into darkness starts with Lois Lane's death, and he is aided by Lex Luthor, who later attempts to kill Superman with Kryptonite.
"A Better World": Superman's first victim is the villain who killed his close friend and taunted him about how his "no killing policy" would mean they'd get away with it. The rest of the Justice League follow Superman into becoming Fallen Heroes and Batman is the most reluctant to go with the whole Knight Templar thing, with him helping the alternate universe heroes to take them down. Superman also plans an invasion of the heroic universe to enforce his own ideals.
Crisis on Two Earths: A heroic Lex Luthor reaches out to the heroes of another universe to defeat a league of their evil counterparts. The heroes are also aided by that universe's Slade Wilson.
For the Justice League fandom, in addition to Batmannote Kevin Conroy, Supermannote George Newbern, Wonder Womannote Susan Eisenberg, Hal Jordannote Adam Baldwin, John Stewartnote Phil LaMarr, the Martian Manhunternote Carl Lumbly, and Killer Frostnote Jennifer Hale retaining their respective voice actors, Lex Luthornote Mark Rolston is Firefly; Cyborg and Doomsdaynote Khary Payton are Ten; Harley Quinn and Ravennote Tara Strong are Queen; Catwomannote Grey DeLisle is Shifter and Downpour; and Barbara Gordonnote Kimberly Brooks is Mrs. Saunders. Batman is also Atom Smasher, Commander Steel, the Crimson Avenger, and Joe Chill; Superman is also Bizarro, Evil Star, Wildman, and D.J. Rubber Ducky; Wonder Woman is also Rampage; Jordan is also Jonah Hex and Captain Rick Flag Jr.; Stewart is also Ed Reiss, Chris McGee, Static, Steel, S.T.R.I.P.E., and Machiste; the Manhunter is also Kryzblack, Py'tar, and Faz; and Killer Frost is also Zatanna and Giganta.
For the Teen Titans fandom, in addition to Cyborgnote Khary Payton and Ravennote Tara Strong retaining their respective voice actors, Barbara Gordonnote Kimberly Brooks is Sarasim; and Lobonote David Sobolov is Cron. Raven is also Kitten, Kole, Elasti-Girl, Jinx, and Gizmonote albeit as The Other Darrin for Lauren Tom as the latter two; and Cyborg is also Hot Spot and Herald.
For fans of The Avengers: Earth's Mightiest Heroes, Hawkgirl and Killer Frostnote Jennifer Hale are Ms. Marvel; Nightwing and Sinestronote Troy Baker are Michael Korvac, Clay Quartermain, Whirlwind, Blizzard, Grey Gargoyle, Ulik, Faradei, Sydren, Groot, Glenn Talbot, Robbie Robertson, and Constrictor; Aquaman and John Stewartnote Phil LaMarr are J.A.R.V.I.S. and Wonder Man; Solomon Grundy and Banenote Fred Tatasciore are the Hulk, the Thing, Red Hulk, Graviton, Mandrill, Volstagg, Fernis, and Yon Rogg; Catwomannote Grey DeLisle is Betty Brant; Zatannanote Lacey Chabert is Quake; and General Zodnote Nolan North is Balder, Jimmy Woo, Living Laser, Piledriver, and Chemistro.
For the Young Justice fandom, in addition to Aquamannote Phil LaMarr, Green Arrownote Alan Tudyk, Lobonote David Sobolov, Lex Luthornote Mark Rolston, and Zatannanote Lacey Chabert retaining their respective voice actors, Cyborg and Doomsdaynote Khary Payton are Aqualad and Black Manta; General Zodnote Nolan North is Superman, Superboy, and Giovanni Zatara; Catwomannote Grey DeLisle is Dreamer and Whisper A'Daire; Hal Jordannote Adam Baldwin is Parasite; Harley Quinn and Ravennote Tara Strong are Dr. Sterling Roquette; and Solomon Grundy and Banenote Fred Tatasciore are Deathstroke and Ubu.
For fans of the Batman Arkham Series, in addition to Batmannote Kevin Conroy, Catwomannote Grey DeLisle, Harley Quinnnote Tara Strong, Solomon Grundy, Banenote both Fred Tatasciore, and Barbara Gordonnote Kimberly Brooks retaining their respective voice actors, Nightwing and Sinestronote Troy Baker are Two-Face, Tim Drake, Officer Whitman, and Origins!Jokernote as The Other Darrin for Mark Hamill in the latter case; General Zodnote Nolan North is the Penguin and Black Mask; Cyborg and Doomsdaynote Khary Payton are Azrael, Officer John Forrester, Warden Joseph, and Origins!Killer Crocnote as The Other Darrin for Steve Blum in the latter case; Lex Luthornote Mark Rolston is Deathstroke; and in Lockdown, the announcernote Amy Carle is Poison Ivy, The Other Darrin for Tasia Valenza. Batman is also Thomas Wayne and Hush; Catwoman is also Dr. Stacy Baker and Martha Waynenote albeit as The Other Darrin for Valenza in the latter case; Grundy and Bane are also Carl Todd and Mr. Hammer Abramovici; and Barbara is also young!Bruce Wayne, Dr. Sarah Cassidy, and Officer Anne Bishop.
For the Diablo III fandom, Catwomannote Grey DeLisle is the female Wizard; the Martian Manhunternote Carl Lumbly is the male Witch Doctor; Hawkgirl and Killer Frostnote Jennifer Hale are Leah; Lobonote David Sobolov is Azmodan; and Nightwing and Sinestronote Troy Baker are Lyndon the Scoundrel.
For the Firefly fandom, Hal Jordannote Adam Baldwin is Jayne Cobb; and Green Arrownote Alan Tudyk is Hoban Washburne.
For the Ben 10 fandom, Harley Quinn and Ravennote Tara Strong are Young!Ben Tennyson, Young!Ben as Upgrade, Future!Gwen Tennyson, and Princess Attea; Solomon Grundy and Banenote Fred Tatasciore are Cannonbolt, Ripjaws, Future!Ben, and Way Big; Cyborg and Doomsdaynote Khary Payton are Hex and Manny Armstrong; Barbara Gordonnote Kimberly Brooks is Princess Looma and Omniverse!Serena; Catwomannote Grey DeLisle is Heatblast!Gwen and Xylene; Batmannote Kevin Conroy is Alien Force!Bellicus and Alien X; and Supermannote George Newbern is Frank Tennyson.
For the Static Shock fandom, in addition to Batmannote Kevin Conroy, Supermannote George Newbern, John Stewartnote Phil LaMarr, and the Martian Manhunternote Carl Lumbly retaining their respective voice actors, Barbara Gordonnote Kimberly Brooks is Puff and Madelyn Spaulding; and Hal Jordannote Adam Baldwin is York. Stewart is also Static; and the Manhunter is also Anansi.
For the Bioshock Infinite fandom, Nightwing and Sinestronote Troy Baker are Booker DeWitt; Hawkgirl and Killer Frostnote Jennifer Hale are Rosalind Lutece; and Barbara Gordonnote Kimberly Brooks is Daisy Fitzroy.
Regime!Superman could have been more Obviously Evil in design, potentially even having Zod's famous facial hair.
Black Canary was potentially going to be included in the initial roster, at least, which would have made her the analogue to Sindel, gameplay-wise.
Concept art indicates that John Stewart could have been used as Green Lantern instead of Hal Jordan. Stewart was eventually added to the game as an alternate skin, with Jordan being used in the game's story mode and the character's classic battle ending.