News tends to trickle. A lot of the time, when you hear about someone's next project, they've developed it to a certain point and are happy enough with it to announce it. For all we know, Johns has been working on it, or is even done with it. I doubt there's no script for it at all, or that writing work on GL is just beginning. Johns was involved in WW, WW2, AQUAMAN, and SHAZAM. Odds are pretty good he's had a hand in GL as well, and has already done some work on it.