Masks for Shape Key Influence

Currently we only have the option to create Vertex Groups to limit the influence of a Shape Key. This is a very stiff method because with vertex groups you can't paint a soft mask or save an image of the influence to transfer it. It would be really nice to have a mask input or to paint vertex groups not just with 1 or 0. Is there an option for that?

I feel like this area in blender was untouched for a long time, but for a good facial rig, this would enhance the workflow a lot.