One issue is camera vibration/movement when the current shutter button is pressed hence why manual remotes are used on tripods for longer exposures. It is possible that IBIS will counter the movement if haptic or deliberately added vibration is used to to provide feedback but removing the source is obviously best. An artificial sound/click could be suitable feedback rather than haptic etc.I'd be interested to know how you would initiate half-press/AF. Maybe it would have haptic feedback and pressure sensitivity like an iPhone home button.
In the current world of touchscreens/pads, it makes sense to investigate this option but the most resistant users will be at the higher end of the market. New M series bodies could be the best starting points for introduction and working out bugs but cost will be higher than the current button.
Lastly, would the touch pad be flat? Are there curved touch pads being used in other commercial devices?