I hope they stray very little from the original Canonet design. A full frame sensor and a fixed 40mm f/1.7 lens would be perfect. Alternately, a 1,5x crop sensor and a 35mm f/1.4 lens. It would be a slightly tighter view, but you not really loose as much freedom with regards to DoF. The viewfinder could be completely from any information for all I care, as long as it is at least as big as on the one on the 5D.