I've seen speculation about a Sony MF mirrorless from a number of sources. One interesting possibility is that it might be a fixed-lens camera (similar to the RX1 series). That seems to make more sense than an interchangeable lens mirrorless based n the 50 MP sensor , given the rest of Sony's product line (I think we can rule out a MF DSLR from a company that has never made any sort of MF SLR, and had never made any SLRs at all until they bought some from Minolta). Both Fuji and Hasselblad made sense for MF mirrorless - Fuji had a huge amount of experience with MF optics (both from the film era, and making lenses for Hasselblad DSLRs), and had a very popular APS-C mirrorless system topping out at 24 MP (with a lot of pro users, many of whom used Fuji mirrorless alongside Canon or Nikon DSLRs) that they needed a "big brother" to. Hasselblad had a big MF DSLR system that they wanted a walk-around counterpart to. Neither was in the high-res FF DSLR (or mirrorless) market that might compete with MF mirrorless.
Sony, on the other hand, is the leader in high-res FF mirrorless, with a 42 MP body that comes very close to the resolution and general performance of the 50 MP MF sensor. The replacement for the A7RII will very likely be OVER 50 MP, with close to MF dynamic range and noise performance (the D850, which uses a Sony-derived sensor, is already very, very close to that 50 MP sensor, and the A7RII isn't far off itself). Spec-obsessed Sony wouldn't enjoy trying to sell a system with only limited extra resolution and much slower performance than their own A7RII, based on less tangible aspects of image quality.
If Sony is going to try to do an MF mirrorless, this leaves them three options if the 50 MP sensor on an interchangeable lens body is too close to the A7RII and upcoming A7RIII.
1.) Shoehorn the 100 MP 645 sensor into a mirrorless body (possible - Fuji's and Mamiya's film-era Texas Leicas weren't that huge, and many of them used formats larger than 645 - there was even a 6x17, although that DID get big). That leaves Sony with a probably hugely expensive body (that sensor has never been seen in anything selling for less than $30,000, while Pentax already had the 50 MP crop sensor in a $7000 DSLR body before Fuji and Hasselblad came along), and a need for lenses with a large image circle.
2.) Wait for the 100 MP sensor the size of the current 50 MP (late 2018?) and do a body based on that. If APS-C mirrorless (where their major competitor is Fuji just like they'd be taking on here) is any guide, the Sony would be the smallest possible body that could hold the sensor, sacrificing some control flexibility. It would be cheaper than the 100 MP GFX derivative, and would probably have more firmware features, but less hardware control, than the Fuji. It might look oddly like the Hasselblad, although perhaps without the jewel-like build quality. Sony might well have trouble getting lenses out for it, which Fuji is VERY good at...
3.) Build a fixed-lens camera, possibly based on the 50 MP sensor, that significantly undercuts Fuji and Hasselblad on weight (and maybe price). This could even be "practice" for an interchangeable lens variant once the 100 MP sensor comes out. Fuji did that with the original X100 (the X-Pro 1 was kind of an X100 with a lens mount), and Sony did it themselves with the Rx1 (which certainly foreshadowed the A7 series).
