Hmm, an evil idea that's neither EVIL/MILC nor FF: Steal the design of the
Olympus Pen-FT half-frame 35mm SLR, with a PK lens mount. Working name: K-Pen-D.
The Pen-FT (and its kin the Pen-F and Pen-FV) shot frames the same size as an APS-C sensor. Rather than a top-mounted pentaprism, the Pens had an internal vertically-oriented porro-prism (like in binoculars), giving it a low, flat-top profile. Because there were no 'legacy' lenses to support, the Pens' Zuikos were designed with a register of 28.95mm, compared to the Pentax 45.46mm. Thus a K-Pen-D would need the mount extended by 16.51mm (~2/3 inch) -- enough body thickness to contain sensor and screen.
Compared to FF SLRs, the midget Pens use 1/2-size mirror and prism; images are portrait mode, not landscape. The K-Pen-D would similarly have a smaller, lighter, faster, quieter mirror, a smaller and lighter prism, and it would easily slip into a pocket. This is without an AF motor, of course. We're talking BASIC cams, right? Optional add-on: battery pack with AF motor, for sluggards who can't handle MF.
It'll never happen, of course.