So if you look over here, it gives you an overview of who the charactes are in the game
Each of the characters has a link on the more info to their page in DCCOMICS official website
This official website started inserting character profiles after last year september during the #0 month and gave everyone a profile on who these people are.
So, what happened was after September, they started inserting several characters on the character page which includes
Ra's al Ghul
Most of these characters are already appearing in the game, my guess is that the remaining characters that would be revealed would be one of the characters up there and also, ofcourse the other characters that are already on the character page and hasn't been revealed
Chances are if your favourite character isn't in DCCOMICS character page, theres almost 0 chance of appearing in INJUSTICE.
Sorry Black Canary, Red Hood, Starfire and Katana. Even though Superboy is up there, I think the chances for Superboy is pretty low as well :(