Looks like the rumor about there being a second set of Injustice DLC characters might be true. Today Ed Boon, creative director of NetheRealm, replied to a question of whether there would be more Injustice DLC character announcements coming with a definitive yes.
A couple of months ago it was rumored that a second set of characters are coming. That DLC set might include Power Girl, Martian Manhunter, Red Hood and Sub-Zero. The last of the first four DLC characters, General Zod, will be available for download tomorrow.