My first camera, in 1953, was a point-and-shoot medium-format camera made by Kodak called the Brownie Hawkeye. 
It used 127 films, and I did my own developing.
I got my Brownie Hawkeye in 1951, but I remember that it took 12 6x6 images on a roll of 620 film.
As we consider the new medium-format mirrorless cameras from Hasselblad and Fuji, two things seem to be possible as we look into the future. First, I would not be surprised if Pentax gets into the fray with a mirrorless rendition of the 645Z. This seems to be a logical next step for Ricoh/Pentax. Then, as mirrorless begins to be firmly rooted in the medium-format realm, the next possibility will be towards greater "medium" formats, such as 6x6, 6x7, & 6x9 were the larger medium formats in the film era. It seems unlikely we'll go beyond that in the remotely forseeable future because Roundshot already does medium-format scans of 6x17 and beyond.
We're already there.
I use, and have for years, a mirrorless camera with a imaging area
larger than 6x9: 7.2x9.6cm. It's a Linhof Technika with a Betterlight Super 6K back.
If you say that scanning doesn't count, you can put a 2D 40x54mm sensor on a tech camera. That's mirrorless, too.
I see the trends heading towards smaller, rather than larger, sensors, as their performance envelope grows to the point where they are good enough for more and more things. We've got "baby MF" with the 33x44mm chips. We've got 50 MP FF chips. We've got FF FWC's and RN levels that exceed those of any commercial MF areal sensor of a few years ago. That won't stop soon.
Onb the flip side, we've got fabs costing more and more, and thus chip area having cost pressure.
Jim
