The first DLC characters have now been revealed in Injustice 2. They are available in the Fighter Pack 1 and features a special guest from Mortal Kombat.
The three DLC characters are Starfire, the Red Hood and even Sub-Zero from Mortal Kombat. Other silhouettes were revealed and we may be getting Raiden from Mortal Kombat as DLC in the future too.
When buying the Ultimate Edition of the game, you will get Fighter Pack 1 plus six other DLC fighters included. The DLC characters are usually released a few weeks/months after the game has come out.
The game will be available on PS4 and Xbox One from May 16th, 2017. As of right now, no PC or Nintendo Switch versions have been announced thus far.