Batgirl was leaked as the second downloadable content for Injustice: Gods Among Us last week, and NetherRealm has copped to the addition. Admitting "the bat's out of the bag," it provided a picture of the Injustice iteration of the character, complete with some pretty vicious looking weaponry.
Shortly after Lobo was announced as the first DLC, a rumor (via Kotaku) started circulating that the remaining three would be Batgirl, General Zod, and Scorpion from Mortal Kombat. Zod makes sense, as it ties in nicely with the upcoming Man of Steel, and Scorpion would continue the Mortal Kombat trend of borrowing from another franchise for the fourth piece of DLC. So far 1/3 of the rumor has panned out, so we'll have to wait to see about the other two.