A good book on the strategy of facial animation is Jason Osipa's "Stop Staring" although I don't think the current version covers A:M and the original version wasn't aware of several relevant features in A:M.
He has varying levels of need-to-have
Some minimums for the mouth would be open "ah" pose, then the sideways stretched "ee" pose, then the "oo" pose and maybe the slightly curled in "mm/b" pose.
Then you might add raising and lowering the corners of the mouth for smile/frown. Note that smiles pull back more than up.
You want to be able to raise and lower eyebrows and at various angles.
You want to be raise an lower the upper and lower eyelids independently to follow eyes looking up or down. Real eyelids are not rigid shells that rotate up and down, they drape over the pupil and the part where the pupil is is pushed a bit wider.
Nostril flares? Cheek puffs? maybe.
My own preference is to rig most of the face features with bones. Things like jaws, lips and eyelids tend to move around a center point and bones lend themselves to doing that smoothly whereas poses tend to move in straight line fashion from key to key.
I find cheeks maddening to do well, to keep the fleshy look as the lips move around. I resort to smart skin to augment the result of my boned lips and jaw.