When you add the already announced characters with the supposed roster leak it comes out to 31 if I'm not mistaken. A number that has been listed recently along the 9 DLC characters so it is very very possible.
I would be fine if it were the case (Swamp thing, Scarecrow, John and Hal being two characters, Mr. Freeze, Cheetah)
And I'd also be fine if it wasn't exactly the case either. With how large a character pool they can pick from in DC it could be even better. I mean, I highly doubt we're gonna get rejects like Bouncing Boy or the Fiddler.