Yet to emerge in the Injustice universe (though, his nemesis Killer Frost would), Firestorm has become a recognizable character again in the public eye due to his emergence in the DC Television Universe. For Injustice, much of the young heroes such as Shazam and Robin were recruited to Superman’s cause (while the Teen Titans were sent to the Phantom Zone by the would-be dictator) while Batman has started to look for new allies in his cause (Superman opting to bring supervillains into his ranks as the Dark Knight eventually drafted the Justice League of a parallel Earth). It would make sense for Batman to bring in a young powerhouse like Firestorm for his side. The addition of Firestorm would also add some much needed diversity to the game.
Only making some cameos in Injustice: Gods Among Us, Deadshot seems to be a perfect soldier to be employed by Superman in his war on Batman (much like Bane in that regard). Between Deadshot’s presence in the DC Television Universe and upcoming Suicide Squad film and his use in the popular Arkham video game series, Deadshot would be a prime candidate for Injustice 2. Further, the use of upgradable armor in the game would work extremely well especially considering the kind of weaponry he may come to employ (machine guns, rocket launchers, etc; making him perhaps more like Boba Fett or War Machine in this interpretation).
A key fighter in Batman’s insurgency, Black Canary would seemingly be killed as she revealed the madness of Superman to the world. She would, however, be revived by Doctor Fate who transported her to another Earth where her counterpart had died so she could live in peace with her son Connor and perhaps begin a new life with that world’s Green Arrow. Good soldiers die hard, however, so it seems likely any peace she might find would be overshadowed by guilt of leaving a world that so desperately needed her. It wouldn’t be surprising then that she would return to save her Earth from Superman’s evil. As the Black Canaries have been a consistent element of the DC Television Universe, the character certainly has the presence to be known enough to the general public to warrant an appearance in Injustice 2 as she would also fit well within the theme of upgradable armor.