By the definition supplied by Wikipedia, medium format is any size larger than 24x36mm, but smaller than 4x5 inches. I remember some time ago, there was a rumour about a 1Ds MkIII replacement with a square sensor and I think a separate rumour about a 50MP range replacement. Of course, there were also various other rumours, including a 32-36MP sensor. A thought to chew on, by my calculation at the time, a 36x36mm sensor, with the same pixel density as a 32-36MP 24x36 sensor would be close to 50MP. Whether that would still fit within the image circle of an EF lens, I have no idea, I suspect it would be tight and would certainly result in some quite impressive vignetting. Something else that may be a possibility, with the 4:3 format of the G1-X, who's to say that Canon won't at some point apply that to a larger sensor DSLR. It might sound a little silly, but by the Wikipedia definition, a 36x27mm sensor would qualify as medium format.
Some quick calculations and you can't really get any bigger than 30.6x30.6 for a square sensor and 34.5x25.9 for a 4:3