I've been thinking about Canon MF for a while now, and also had this thought at one stage too.
First problem is, how big do you make the sensor? I've made a mock-up drawing of different sensor-sizes attached below.
Assuming an EF-lens properly covers the FF image, it's got an image-circle diameter of 44mm (in reality not true, a lot of faster lenses vignette a lot before this, TS-E by definition covers a lot more).
Medium Format 120-film square is 56x56mm (but i've shown the 54mm which is the biggest used by digital backs).
I've also shown a 44mm-square sensor, that would just cover the image-circle all-ways.
Now, flange distance of an EF lens is 44.0mm, flange to sensor. 24mm of this (FF body) is taken up by the mirror, leaves 20mm. The distance from the flange to the inner-most bit of an EF (not EFs) lens is 7mm (just measured my 50/1.8II), so that's 13mm left. The sensor sits behind a few protective layers, IR and Bayer filter, whatever, take off another 1mm for that, 12mm. Add in a bit of safety-error-margins (you don't want your mirror accidentally scraping your sensor, or your lens), and maybe you can get a 32mm-high mirror in there. (You can make it as wide as you want, up to 54mm or bigger). So you'd only get an image sensor of 32x44mm or so (which i've added in dashed lines), without increasing the flange-distance.
Or, you can can increase the flange-distance to a normal 6x6 MF distance (74.1mm for Pentacon Six up to 112mm for RB67). They did it before with the move from FD (42mm) to EF (44mm), but lenses won't focus at infinity any more. So you'd need a piece of glass in the adapter, like a mini-teleconverter. Proper Canon-brand FD-EF adapters from the 80s were good quality, and now sell for a lot of money. Or you can buy one from china on ebay for the price of a jar of jam (the bottom of which is probably where they got the glass for them).
Not saying that it won't happen, but the technicalities in making an MF camera are big enough without trying to adapt EF-mount lenses onto them.
(Another one is that the metering will not be accurate if there's black-corners pushing the exposure upwards, that's fixable with software)
If canon bring in an MF-camera (and I hope/wish they would), then you're looking at a whole new range of lenses, on a new lens-mount. But it'll be expensive, probably a lot higher than the "cheap" â‚¬10k pentax 645D, maybe up into â‚¬15-20k Leica S2 and Hassy-territory. Then canon would have a lot of competition from companies who have been in the business a lot longer (try 60+ years for hasselblad). Any pro wanting to switch, and having to invest in a whole new system and lenses, would be idiots for not considering pentax/leica/hassy/phasemiya. Only accessories like flashes and remotes (and DPP) would be interchangeable between the systems, not enough to sway most people on their own.
That said, canon do have a lot of lens-making experience, I don't think i've ever seen an MF lens with IS for example, and never uderestimate the power of fanboys switching formats but not brands.
One more option, in keeping with the EF-mount-on-MF-sensor, how about a Medium Format Mirrorless (Then the flange distance can be even lower, like 20mm, because there's nothing between lens and sensor)? I'll believe it when I see it, even an APS-C mirrorless would be a start...