Well first let me just say, this could be just a rumor, but at the GameSpot.com Games Discussion forums, a user posted -
"I can confirm the first 4 DLC Characters for Injustice!! This is 100% real, I have actual source codes for the game. They are as follows:"
1) Lobo
2) Batgirl
3) Zod
4) Scorpion (Mortal Kombat)
Why add Scorpion? He has nothing to do with DC Comics at all. What do you think?