We already know that Lobo will be Injustice's very first DLC character. It's also a safe bet that he won't be the last, though it's anyone's guess as to who will is next in line.
Then again, perhaps TechTudo has some kind of inside track? According the Brazilian website, which Kotaku brought up over the weekend, they've identified three other characters that will be making guest appearances eventually. Hopefully.
First there's Batgirl. At the moment, the men out number the women to a considerable degree, so simply having any new female face is very much welcome, though it also helps that Barbara Gordon is compelling character in her own right.
Next is General Zod. Most people (who don't read comics) know him as the primary bad guy in 1980's Superman 2, though he's also part of the upcoming Man of Steel, later this summer. It'll be interesting to see if NetherRealm makes him more like Terence Stamp or Michael Shannon (the actors who play Zod in the respective movies).
And finally, somewhat out of left field, but not really, is Scorpion from Mortal Kombat. Given the developers, it was pretty much a given. Plus, he’s mingled with Superman and company once already, via Mortal Kombat Vs. DC Universe.
Though given that Injustice sports no fatalities, will it be the same without the undead ninja in yellow's ability to rip someone's head from their body, with the spine attached?
Anyhow, only time will tell if we actually see any of the three in reality.