Add specular control in the material
I have some situations where I want to control the specular level in a material besides the roughness and metallic. There a cases like a fabric with a decal on top that you need to reduce the specular on the fabric and increase where is tinted, that is different from the metalness or roughness.
At the moment we are going with a PBR metal/roughness system which does not expose specular value. This may change in the future but is currently not planned.