If Canon can make a camera like this with an "affordable" pricetag (around 3.5K instead of Leicas 7K), it will be a huge success! Especially among people who wants to go mirrorless without sacrificing image quality, thin DOF capabilities and high ISO performance.
The biggest plus of FF mirrorless would be that you could adapt pretty much any lens that has ever been made and get the full fov. APS-C and m43 take normal length and tele lenses quite well, but the options for adapted wide angle lenses are limited.
Other than that, it would have the downside that it would mean the native lenses would all have to be larger, so the camera would be larger and heavier than m43, Sony or other contenders.
The Leica is typically used with normal length primes. If you want to use a fast zoom, it will be big (the m43 12-35mm f/2.8 zoom is 300gm)
Anyway, this strikes me as a bit academic. My money is on Canon releasing two slow zooms and a wide pancake prime. I don't understand why anyone would want to "wait" for Canon. They are just too far behind at this stage to be compelling. m43 have the 25mm f/1.4, the 45mm f/1.8, 20mm f/1.7, 12mm f/2 a 45mm macro, the panasonic 7-14mm and now the 12-35mm f/2.8. Canon won't have anything close to this.
Being able to adapt lenses is fun, but it's not a satisfactory substitute for having a good selection of native lenses. Unless the native lenses are good enough to cover the majority of shooting scenarios, there is little reason to prefer the mirrorless over a native (EF mount) body