As I said a couple of days ago, Rose Guns Days is going to have 4 seasons. We should expect season 3 for the summer comicket in August while season 4 will be released at the end of the year during the winter comicket if nothing unexpected happens.
Today, Ryukishi has written this on his blog:
EDIT: I’m going to edit this just in case but as you can see, Ryukishi isn’t really saying that there’s a new series. But if we consider that he talked that he had another project in mind after finishing RoseGunsDays and above it says that he is working again on some plot for When They Cry it would be strange if in the end we won’t have a new series.
Better keep the hype until an official confirmation for now. I’m sure it won’t be long until Ryukishi says what is going to be his next work.