It would require them to individually custom animate every face's 3D mesh to it's absolute extremes.
If it were to be implemented properly it would probably have to be available only for the default skins and a small selection of popular skins. And that's if they even have the tools ready to push facial animations like that at all, idk if there are limits to the current rigging that would need to be addressed, idk how they'd do it tbh.