Maybe we are going to have the equivalent of metabones to adapt L lenses on the canon medium format...
As for the bodies; if Canon opts to buy Phase-One or some other producer, it can easily inherit that company's existing line of lenses. If the body system it supports has a longer flange distance than the EOS system, then that's great; because then Canon can use .5x wide-converters mounted between the lens mount and the EOS lens to adapt the lens for the MF camera.
Impossible, a wide-converter inherently requires a shorter flange distance than is native to the lens (unless someone knows differently). That is the reason that the Metabones uses only SLR lenses with only mirrorless cameras (not APS DSLR cameras).
Also, there are multiple digital medium format sizes:
44 x 33mm (Pentax 645D, Hasselblad - multiple, Mamiya Aptus 8 & Credo 40)
45 x 30 mm (Leica S2)
48 x 36 mm (Mamiya Aptus 5 & 7)
49.1 x 36.7 mm (Hasselblad - multiple)
53.7 x 40.2 mm (Hasselblad H4D-60, Mamiya Aptus 12 & Credo 60 & 80)
56 x 36 mm (Mamiya Aptus 10)56 x 41.5 mm - Film '645' Format70 x 56 mm - Film '6x7' format
Essentially, digital has not yet quite made it to the medium formats everyone refers to from the film days. The Pentax 645D doesn't have nearly the same advantage over FF cameras as 645 film did over 35mm.
Size and weight comparison:
Canon 1D X Canon 5D Mark III Leica S2
1540g 950g 1410 g
158 x 163 x 84 mm 152 x 116 x 76 mm 160 x 120 x 80 mm
So the Leica S2 is slightly larger than the 5D Mark III (although almost 50% heavier), and lighter than the 1D X (and much smaller). Medium format doesn't have to be huge like a 120/220 camera as Leica has shown (although, again the sensor is smaller and can therefore have a smaller mirror and a shorter flange distance than the film formats).