Hopefully it's going to be a -Medium format / Full frame mount.
That was the first thing that came to mind for me, that Sony CMOS MF sensor does seem to make it more likely that another big player(Sony, Nikon, Fuji, etc) might give MF a try which could well have Canon looking to respond.
If it were a FF mirrorless mount(can the EF-M mount take FF? I remember seeing some analysis that it could) my guess would be that Canon would come up with something similar to the EF-M setup at the moment. Maybe a body that offers a bit more control but generally a system aimed a keeping things small with a handful of compact lenses rather than a full system which would be kept for DSLR's.
One thing I'm starting to wonder as well is whether the cheap EOS M stock in the west isn't just as was first suspected a case of Canon selling unwanted product but rather a direct plan to devalue the mirrorless market? how long has it been since the big EOS M price drops? almost a year? I can see the logic to doing this as it avoids pushing DSLR prices too low whilst Canon don't have to recoup a relatively small investement(by the standard of other companies) in mirrorless. Could they try something similar with FF? introduce a basic beyond below the 6D.