In the examples below you can see why some basic examples need to happen eventually. the chars look like from a decade ago w/o any form of facial expressions.
maybe start with blinking and later squinting when having heavy weather or when having dusky/dawny light.
maybe basic good mood (fit and healthy) expression, basic bad mood expression (wounded) or focused expression (aiming, shooting).
maybe it's possible to make the eyes follow one's mouse cursor, so that others see what/where you are looking at.
examples from the latest exp update: