3d tracking in a face without trackers is like chasing God with lightspeed. In other words impossible.
more easy is to make Grievous darker and less yellow and cover his eyes with lenses rather can we expect to replace him totaly with something new or a new head as well.
Lets be realistic.
Ideas all all good to come but you can always think if its possible to be done ( with the present tools of trade and fan knowledge)
-Angel