Regarding displacement, I suggest the following extension. This one is for geometry displacement. If another displacement technique needs to be defined e.g. parallax, a displacementParallax could be defined.

{
  "materials": [
    {
      "displacementGeometry": {
        "displacementGeometryFactor": 1.0,
        "displacementGeometryTexture": 0,
        "displacementGeometryOffset": 0.0
      }
    }
  ]
}

From the texture, the r channel is used. The factor is used to multiply the channel. If no texture is present, the value is take as it is. Finally, the offset is added to the above value.