It would be funny if Canon has a very different idea of retro to us. It might only be as far back as a T70, those FD lenses didn't look too different, polycarb shells, different rubber ware and of course an aperture ring.
They could go VERY retro into their film range finder territory, like the old Canon P or 7 series, but mounting a modern RF lens on these camera styles would look very odd and those old Lecia thread mount lenses are very different in size and shape to modern optics. However, having an offset EVF might be fun to use. It would be fun to have a EVF only UI...so no back LCD screen would look VERY old skool.
Canon might consider the EOS 300 / Rebel 2000 as retro small film and those were EF mount lenses, not too dissimilar to the current RF shells, easy to re-imagine a few RF lenses in that style. Something like the EOS 33 / Elan 7 were the a lot bigger and were in some respects the forerunners of the 5D range (ie not the 1 series, or the 3 series, but the next 5 series model).
I guess it depends on what Canon means by "retro". I can't imagine a EOS 650 sized camera, even a AE-1 will be bigger than the current R8 foot print.