The Fuji X Half camera is what led me back to the Leica X2, which is as close to the Olympus XA (and a number of other compact 35s) as I've seen. The Leica's collapsing lens makes it quite pocketable, the controls are simple and easy like the XA (and others) and if you can find one of the self-opening/self-closing lens cap gizmos that were made for it, it's as convenient as the sliding closure of the XA (no lens cap to fuss with).
A FF format digital camera in this form factor is difficult to imagine happening as there seems to be very little market for one. The Sigma fp is close considered without the lens, but even a compact lens on it makes it a bit bulky.
G