According to a recently interview with Sony engineers and managers by Photolari. Sony told us that currently they didn’t have any plan for rumored Sony Medium Format mirrorless camera yet, because they are too busy for the full frame mirrorless cameras. The next full frame mirrorless camera from Sony will likely be Sony a7III, it will be announced in early 2018.
Sony claims if they decided to launch a medium format camera they could do it, but that now is not the right time. The launch of a medium format camera would require Sony to build a new ecosystem and they are quite busy right now building out their full frame camera system.