Release of the Ethiopian and Lion Air crash reports may very well be the elephants in the room.
With the animosity Boeing and the FAA have managed to muster, it would not surprise me if one of more NAA would want to keep the aircraft grounded until the cause of the accidents has been properly identified via an official accident investigation report. Which would bring the grounding well into 2020, while the actual length of the grounding will be, at least partially, decided by how many objections Boeing and/or the FAA will lodge against the findings prior to to the official release.