I guess a good example for what I want is almost like a south park character and their mouths.
If I had a sphere for a head, and I wanted to draw a mouth on it with grease pencil, I'd draw 5 difference mouth shapes across 5 keyframes with the grease pencil object. Is there a way to link this to a controller or slider, that I can move up and down, to move through the keyframes for lip syncing? That way I could just include it with the character rig instead of having to redraw the mouth in every shot.
Or if I had a plane of Cartman's head and I drew his different mouth shapes as a grease pencil object, then i give it to an animator so they can just use the slider to go through the mouth shapes and keyframe that slider.
Same could be done with eyes for example. Instead of making a layered rig with bones, if I could just draw the eye open, closed and maybe squinting and connect that to a controller that slides through each keyframe for each eye shape that would be what I want.
Hopefully that makes sense, I cant find anything related to this or it is hard to find the right keywords to search. Do I need to write some python script for it?