So...
Custom curve is easily doable using falloff node
The same node also allows non-physical artistic control of fresnel
Fresnel IOR in CoronaMTL allows for same PBR control, just in opposite direction
PBR mode would allow the same thing in the direction of control you request
Reflection level allows another layer of non physical fine tuning
So currently, there is all flexibility needed, and it's just about reducing it to guide people to create more physically based materials.
One of the ideas on the table is to create layering system, like in Blender Cycles, Octane, or what nVidia does with MILA. Of course, Corona implementation would be on par with Corona usability standard, so idea is to take best of all there is currently on the market, and wrap it into nice polished simple to use solution.
It would have following benefits:
CoronaMTL would stay uncluttered, and friendly for those who need to create quick simple materials. Even for those who create complex materials, it would serve as a material for objects out of area of interest. I doubt many would want to spent time creating layered shaders for distant background objects.
Since there would be basic components like diffuse, specular, scatter, refraction... every node could contain a bit more, and there could be nodes for specific worfklow. There could be for example legacy specular node with same system CoronaMTL has now, and PRB specular with roughness instead of glossiness, inverse Fresnel control, and BRDF picker to pick between ashikhmin and GGX.
GGX is probably necessary for the inverse control Fresnel model, because inverse control implies that max. reflectivity value is always 1. If you think about for example dusty old worn asphalt road, you can hardly imagine such material having maximal reflectivity at grazing angles. This is what GGX solves, because it's a microfacet model that takes self shadowing (and therefore light attenuation on the surface) into account so reflection would correctly dim with roughness.
It would of course give corona so much demanded layering system. There will of course be legacy style multilayer blend material too.
It would bring Corona closer on the same page with new PBR standards.
It would make it probably easier to implement experimental shading models, as when material is separated to basic shading elements, UI of individual nodes is a lot simpler, and therefore we can fit more things in without making it appear overwhelming.
BTW if anyone could get a screenshot of how Disney material UI looks like (the one their artists use day to day), it would help a lot.
Non the less, it's not gonna happen too soon, as there are other a lot more pressing things on the priority list.
