So in the Gamespot article they reported that there would be AT LEAST as many characters in Injustice as there where in Mortal Kombat which shipped with 28 characters. With only Batman, Superman, Flash, Wonder Woman, Solomon Grundy, and Harley Quinn announced that leaves at the very least 22 characters left. This gives me ALOT of hope for this game since they will really have to pull from beyond the mainstream to fill out such an expansive roster.
Here are some characters I hope are in the game
Lex Luthor: All but confirmed. One of DC's most popular characters and he is seen in the trailer in his power suit
Captain Cold: Flash is getting ice shot at him in the trailer. VERY likely
Green Lantern (Hal Jordan): One of DC's most popular characters and he was in DC v MK. Positive he will be in it
Sinestro:(alternate GL costume would be awesome)
Robin: (probably Tim Drake unfortunately since Damian is still too young)
Any and all of these characters would be awesome in the game!