Watching in horror as Hal Jordan was corrupted by Superman and Sinestro into becoming the Yellow Lantern, Carol Ferris has seemingly yet to emerge in the Injustice universe as the Star Sapphire. Alternatively, the Red Lanterns would emerge and seemingly align with Sinestro despite their leader Atrocitus’ hatred of the villain in the source material. The presence of the Red Lanterns (and prominence of the Sinestro Corps) lends credibility to the Star Sapphires appearing as the reversal of Atrocitus aligning with Sinestro could then mean Carol Ferris assuming the mantle of Sapphire in order to combat Jordan and try to bring him back to the side of justice. It also certainly wouldn’t hurt to add another female character to the mix of Injustice 2.
When the mother of John Constantine’s daughter Rose is killed during the Green Lantern Corps’ war on Superman’s regime, the occult detective helped organize the Earth’s magic community to align with Batman’s insurgency. While Constantine acted as some sort of secret agent putting the various pieces in place in order to protect Rose from this new, dangerous world, Doctor Fate largely became the face of the magic community as Mr. Mxyzptlk went about wiping out Earth’s mystical champions. Following Constantine’s treachery that eliminated Mxyzptlk and Trigon, Fate and his fellow sorcerers seemingly abandoned Batman’s crusade (save Zatanna). Dr. Fate has a very iconic look and a power set that would work great within Injustice 2.
Within the various comic series covering the five year span within the continuity of Injustice: Gods Among Us, Batwoman plays an integral character within Batman’s insurgency. Standing by his side throughout that time period, it’s likely Batwoman may have been passed over in the game due to the presence of several similar characters appearing already (Catwoman, Harley Quinn, Batgirl, et al) as characters like Huntress and Renee Montoya (who did not become the Question in this continuity) sadly lost their lives in the battle against the regime. Injustice 2 may see Batwoman emerge to fight alongside Batman taking advantage of the new capability to wear upgradable armor.