Man, I'm groping here. I've been surprised by Cosina every time.
-- More lenses in M mount, so we don't need screw-to-bayo adapters?
-- A body with a longer RF base, now that Zeiss Ikon has pointed the way?
-- A body that can accept an electric motor? Wait, I heard Mr. K doesn't like motors.
-- A body with enhanced metering, e.g. a spot meter covering a defined area?
-- A reflex housing, maybe one accepting Nikon F or other standard SLR mount lenses? (Not sure there'd be enough depth for this, but it's an idea.)
Or maybe it's time to revive the Contax and Nikon mounts, on the more recent upgraded bodies. Using your Contax-mount lenses on an auto-exposure camera would be an interesting notion, would it not?
Okay, now the one I'm really, really hoping for: a good, modern, true zoom accessory finder. Advances in acrylic optics have made it possible for even a cheesy point'n'shoot to have a bright, sharp-edged zoom finder; why not one as an accessory? I'd like to see one with a range of, say, 28 - 100mm, with parallax compensation, and a dial that would adapt it for standard 35mm, Leica M8 1.33x, or Epson R-D 1 1.53x coverages.
And as long as I'm dreaming, how about building in a "VC III" meter to offer through-the-finder, variable-angle spot metering? I'm not sure who'd buy such a thing (other than myself) but it's technically feasible...