Hi Dan,
So, I will look at new sensors that are going to be released this year for medium format and that they were already announced by Sony:
https://photorumors.com/2017/11/11/more-info-on-the-new-sony-100mp-and-150mp-medium-format-back-illuminated-sensors-with-4k-and-8k-video-capabilities/ . It is expected that we are going to have a 150 megapixels digital backs from Phase ONE and Hasselblad and probably an updated version of the Fuji GFX and Hasseldblad X1D of 100 mpx.
Now, if you take that sensor of 100 mpx and size of 43,8 mm x 32.9 mm, if I did my math correctly, if we cut that in sizes of 36x24mm (35mm format), that will be around a 60 mpx sensor.
I would expect that something in the order of 60 megapixeles could be released in the future (it does not need to be an exact scale... I suspect that Sony will like something that allows a good full sensor readout for video in 6k or 8k... ).
Now, if it looks like they have the technology ready to be released by the end of the year in medium format? Would that mean that they release the equivalents in FF? I think we need to consider other factors. Each sensor design cost a lot of money. Look at the 42 megapixel sensor of Sony, they released already in three different cameras (A7r II, R1X II and A7r III), the III is an updated version, but probably an small evolution of the original one, so they can save money recycling the initial design investment. Nikon looks like they are doing the same, D850 sensor is recycled in the Z7 sensor (updated, but I wouldn't expect that are going to have that big differences). In ASP-C it is quite common, the 24 megapixel sensor shared between Nikon, Fuji and Sony cameras, or the 20 megapixel one in m43 shared by the latests Olympus and Panasonic cameras, so the initial design costs are payed by everybody.
If next year or in 2020 we see a FF sensor in the order of 60 or more megapixeles, I will expect that design to be kept for the next 4 or 5 years. Again, to recover the investment.
Of course they are special cases, like the sensors in Nikon D5 or Sony A9 (although I suspect that part of the design is the same in A7 III, just slower) that make the cameras more expensive, just to pay the costs of the special design that can not be as recycled as the other ones.
Now, both rumors for Fuji and Sony new ASP-C cameras point to a 2x something sensor. For what I read, I will expect they are going more for the speed than increase of resolution. They are focusing more in the 20 fps than other characteristic. That will make a very appealing camera for wildlife and casual sports photographers. I think they will expect to sell more to those crowds (like the Nikon D500) than an increased in resolution.
Ok, I think I went sideways in my comments... my main conclusion, they are milking the sensor design for several years to get the maximum return of investment before the next jump that will make people more interesting into buying new cameras.
P.D.: I left Canon a bit out of my comments, because their sensors are used in their own cameras, but they are doing similar things, the 80D sensor was also used in the latests EOS-M cameras for example.