I got the idea for this from watching Blue Eye Samurai. The rendering in that show is largely reliant on good, stylized texture work, but there are times it seems to be much softer or harsher depending on the mood. This could just be information from the light objects themselves, but I realized it could also be a set of shader perameters that allow fine control over how "toon" shaded a material is.

The node group intakes two shaders, one for a base that draws the shadow and main color, and one for the highlights. There's also a fresnel inside that creates stylized rim lighting.

In total there's 4 color inputs: Highlights, Rim Lights, Base, and Shadow. Each can be customized completely, and also each get a setting that lets them blend with the light color. Each can also go from blending smoothly or sharply, meaning the highlight could look like it's toon shaded, while the base color and shadow blend.