Ok guys and gals, here are the top 5 most likely possibilities I think it could be:
1. Joker
(This one should be fairly obvious; he is "the" Batman villain. Also, since Harley is in the game and we know she still holds a giant grudge on Supes for killing her "Puddin," it wouldn't be too surprising to see the Clown Prince of Crime make a return. He is also one of the character's that could potentially receive a complete redesign (like Wonder Woman) since his moveset from the first game was considered garbage by most, especially his trait.
2. Green Arrow
Even though he is dead in the Injustice Universe, Oliver Queen is till a fan favorite character comparable to that of Martian Manhunter. Since Black Canary, his love interest, has made it in, it would only be fitting to have Olly using "cupid's bow" on the love of his life and all those who threaten her too.
3. Catwoman
Batman and Catwoman are constantly hitting on each other, well, mostly her. In the first game, we found out the two of them were actually an item but Batman rejected her affection after she joined Superman's regime. This could possibly be her chance to reunite with Bruce and make up for her past mistakes.
4. Star Sapphire
Since we know Green Lantern is in the game, we could potentially see another "Lantern" character in the form of Star Sapphire. For those who don't know, Star Sapphire, AKA Carol Ferris was once Hal Jordon's girlfriend and since Boon said there will be plenty of "girl power" in the roster of Injustice 2, we may very well see the violet lantern corp make an appearance and with it, Star Sapphire herself.
5. Zatanna
Call this one a little hopeful but since Zatanna along with Doctor Fate played a strong role in helping Batman recover from his encounter with superman (that led to alfred's death), so we may be "believe in magic" once again. Zatanna, like Catwoman has had an interest in Bruce, only this has been developing since the two were childhood friends. At one point, she even used her magic and mind wiped Batman, but despite this, Batman does call upon her for help. The two are always having an "on and off" type of relationship as you can see. The only reason why she may not show up is because she was a previous dlc character in the first game.
