Something I haven't seen in anime-face costumes so far is soft masks, or perhaps I should say, fabric-covered masks. The modern masks are uniformly paint-covered opaque hard plastic or resin, with opaque eyes, and small slits for vision.
I also haven't seen any work done in making anime faces which can move. If you want anything approaching a movable mouth, the options are fursuits or uncanny-valley latex/silicone face sculpts. Neither work for anime. One isn't human in shape; the other is too human.
This has resulted in a new project being added to my buffer:
A plastic wireframe mesh face for a kig head. Probably based on a model from Thingiverse, pierced by an array of hole-defining intersecting shapes.
Fabric stretched over that mesh, arranged in such a way that the mouth can open (at least a little), carefully stitched to the frame for the points of negative curvature, opaque enough to avoid being seen through, yet thin enough to see through. This may require blocking fabrics on the inside to selectively limit throughsight.
Internal ligatures to limit the movement of the face.
A side advantage of developing this style of mask is that it solves the issue of matching mask color to hadati color, in all lighting conditions: just use the same fabric!
But first, I should actually finish the prototype metal mask for Turaga Vakama.