Junya C Motomura's gave an amazing GDC presentation of how Guilty Gear Xrd achieved it's incredible 2D look. And I was inspired to write a variant of what Junya presented as a Maya Cgfx Shader. Check it out, it's available on Github:
https://github.com/miketon/Maya_Tools/tree/master/Shaders/Toon_xrd
Notable differences :
* Team Red appears to be straight up using a dot product. This shader uses a dot product to do a uv lookup on a 1D ramp texture. Affords more flexibility with changing light-to-shadow palette via texture edit. Tradeoff is lower perf
* Team Red actually altered the normals, I'm using vertex color (red channel) to alter the uv lookup.
Future improvements could include other 2D effects such as ziptone and crosshatch shading.