Okay, it's good to see that this game will be a direct sequel to IGAU, but when a new game comes out involving something like comic book characters has a big overhaul it almost seems impossible to just leave out.
DC is in the midst of something huge for the company. Injustice brought TONS of new fans to DC (myself being one).
Watchmen has a VERY good reputation of being one of the best graphic novels ever written. So to me, when one of those characters finally gets a chance in the main DCU, (Speaking about Dr. Manhattan) it just seems unlogical to pass up. Even if he has nothing to do with the story and is a pre order DLC. (Just like Goro in MKX, no story involvement)
This is my best guess, but with the characters they have announced so far I have never heard of, although I am not a huge DC fan, you never really know what NRS is thinking.
(This means I would play both Pre-Order DLC characters)