For those that are fans of NetherRealm Studios or DC Comics, Injustice: Gods Among Us is a series you're probably already familiar with. However, something that players might not know is that the Injustice series spans beyond that of just the games. In order to give the games more backstory, a series of comic books were released which explained the events leading up to the events of the game.
In the comics, a lot of DC characters are introduced that do not appear in the mainline games. The majority of the reasons why certain characters don't appear as playable characters are because, well...they died. However, in the world of comic books, being "dead" usually never amounts to much and characters are back before you can utter "Lazarus Pit" or "alternate reality." With this workaround in mind, let's consider which characters (dead or not) may make a good addition to an Injustice 3.
John Constantine
Constantine gets dragged into the Injustice conflict shortly after Superman decimates the Green Lantern Corps. One unfortunate Lantern Corps corpse crashes into Constantine's house, exposing Rose Constantine and killing her mother. Since that moment, Constantine vowed revenge, as Superman's insane war had cost his daughter not just the life of her mother, but her childhood. Constantine mainly worked behind the scenes, incapacitating many of Superman's allies using his magic abilities. For example, Constantine managed to capture and subdue Raven and convince Trigon that it was Superman's fault. By working with Ragman, Constantine almost managed to take out the Man of Steel himself but was interrupted by Shazam.
While Constantine may not be the best in a physical fight, his wit, grit and overwhelming magical abilities would make him a worthy combatant even in the Injustice universe. By his own words, Constantine has claimed to have taken down "monsters, demons, and even gods." Constantine's words are no mere idle boast, as his actions in the comics have proven his ability over and over.
Plastic Man
Plastic Man is often treated as mere comic relief, but he's pretty powerful in his own right. Even Batman holds Plastic Man in high regard, describing him as a man who has single-handedly infiltrated and taken down some of the most secure facilities in the world. In the Injustice Comics, Plastic Man and his son, Luke, help Batman's team break into the League of Shadows and even Superman's newly reinforced Fortress of Solitude. Plastic Man was also the sole instigator in releasing all of the prisoners Superman had kept in the Trench.
Other than his phenomenal breaking-and-entering skills, Plastic Man possess some of the most powerful abilities in the DC universe. He can change his body to any shape and is virtually impervious to any type of damage. Additionally, his unique physiology allows him to change his body density, allowing it to be as hard as steel or as malleable as putty. With so many tools at his disposal, it would be interesting to see what kind of move set he might have in Injustice 3. With Plastic Man's silly temperament, it would be no surprise if many of his moves were based on classic cartoon tropes, like giant anvils or hammers.
Captain Atom
Captain Atom is an extremely powerful individual with the power of more than ten nuclear bombs residing within him. He was one of Batman's strongest allies in the fight against Superman and his regime and contributed to many early operations. Unfortunately, he met his end when Wonder Woman's magic sword breached his metal suit, causing Captain Atom to release all the nuclear energy within him in a massive explosion.
Captain Atom stands as one of the strongest superheroes in the DC universe. This mainly lies in his ability to freely control and manipulate all forms of energy, especially nuclear energy. This means that Captain Atom can release blasts of nuclear energy at foes, prevent foes from using their powers, and even "detonate" himself, creating an earth-rending explosion. As an added bonus, if Captain Atom absorbs too much energy, he can propel himself forward or backward in time. If NetherRealm Studios does decide to include him in Injustice 3, it would be fascinating to see how they stay true to the character without making him broken overpowered.
Beast Boy
With Cyborg, Raven, Starfire, and Robin in the main games, many DC fans were wondering what happened to the plucky, fun-loving teen titan Beast Boy? It's unfortunate to say, but even in the comics, Beast Boy was just another early casualty. When the Joker's nuke went off in Metropolis, Beast Boy and many other superheroes were caught in the blast. Superboy tried to shield Beast Boy from the nuclear explosion but failed to save him from the radiation.
It's a real injustice that the main Teen Titans that we've grown up with have made a playable appearance in-game, but Beast Boy hasn't. Beast Boy would make a unique addition to the cast, as his ability to transform into any animal would produce a distinct and ferocious move-set. Just imagine turning into an enormous woolly mammoth and simply sitting on Wonder Woman.
Alfred
Now before everyone starts talking about how Alfred isn't a fighter, recall that some aspects of the Injustice series could potentially make Batman's butler into a real fighter. In the first Injustice, mere mortals like Batman and Green Arrow could stand up to super-powered meta-humans due to a super-durability pill created by Lex Luthor. In fact, it was Alfred who first demonstrated the potency of these pills by absolutely obliterating the Man of Steel in the Bat Cave. The rest of the series was not kind to Alfred, as Victor Zsasz later killed him, though he was subsequently brought back to life with the Lazarus Pit. Eventually, he leaves Wayne Manor after deeming himself unfit to serve Bruce Wayne.
Believe it or not, Alfred does have some fighting experience, as he previously served in the army as a field medic, and later as a member of MI-5 before his eventual retirement. Paired with the super pills, Alfred could potentially serve up some considerable punishment to his foes with the crisp, efficient ferocity only a British butler could dole out.