For me, something the size and build of an Olympus XA, maybe with a more direct manual control for focussing, would be a great model for a compact digital camera.
Having said that, being able to reuse old lenses would be tremendously useful? The glass in a lot of old lenses is still superb and competes extremely well with modern lenses.
Control-wise I prefer having manual controls that are accessible on the body and not menu-driven things. Camera design evolved over decades resulting in machines like the 1970's Japanese rangefinders which were easy to operate once you knew what each bit did. I suspect that the radical change in compact digicam design has less to do with ergonomics than with marketing or engineering decisions - perceived convenience over function.
Optical viewfinder? Yes please! When I take a picture, I press the viewfinder so close that the camera rests against my head which makes it much more stable than being held just by two hands.