I am also a big fan of having some sort of ggx or microfacet shading model. imho these are key elements needed when creating photo-realistic materials. I've seen far too many architectural renderings where the floors/walls are reflecting overhead lights and they don't have this implemented-which is a dead give away it's cg. This implementation would also put Corona ahead of the curve in more areas :)
Maybe it doesn't even have to be a new material or customized phenomenon, but maybe set up in a similar way that Mental Ray is doing it's MILA layering library/Iray MDL materials. I don't want to seem like I am praising any other renderer here, since we all know just how amazing Corona is, but imho Mental Ray is really impressing me with how it is now handling the material process now and is ahead of the curve in this area.. for once. With the mila/mdl approach, I can combine tons of different reflection/glossy lobes and do not have to double the rays needed each time I add a new layer like I do with most other renderers, and it all fits into one easy to use phenomonon that doesn't need tons of blend nodes to hook everything up. If I wanted to, I could even have a bunch of glossy lobes layered over an SSS material all within one phenomenon and not incur massive render time penalties. This also makes sharing materials much easier :) (Obviously there are tons of other issues with MR atm though)
...All this to say is if we are thinking of implementing a ggx/microfacet shader, it would be amazing if it was customizeable and not locked into a set material phenomenon where we have to choose between a Corona Mtl or a ggxCorona Mtl... But I don't even know it's possible since there are so many specifics to a ggx material that are required to keep it physically plausible.
To sum all this up.. Layering library methodology is awesome, lightweight and incredibly powerful. Tons of different separate materials scattered everywhere is not so fun.
Lots of respect, and amazing work, as always Keymaster... Looking forward to seeing what you do next!