It will indeed be possible to use any such nodes from the the gui. As for specific attributes and behaviors for them, that’s just a matter of designing the node and deciding what it should be capable of doing.
Whether we disagree about the core types being fragmented, over time it will become less and less necessary to use them directly. Even now, in working on file import for the gui, I am actually not directly using conductor/dielectric/layer at all, since for example, the plastic material’s color, roughness, and specularity attributes give me enough control.
An early request was something like arnold’s standard surface, and that is something we can (and intend to) build. But as I think I have described before, that is not a more flexible material, it is one that hides flexibility, doing internally, what you would otherwise have to do explicitly.
Before that, though, we are working on a PBR-type material, which is intended to allow making use of commonly-available texture sets with a minimum of setup.