At the top of my head, parameters like fullbright shading and additive transparency would be pretty useful to have, in case someone wanted to have a illuminated light source or a glowing fire effect. Some kind of reflection shader would also be nice to have, though I don't know if it'd be possible to implement matcaps (basically something like the reflection texture that Metal Mario uses in SM64) into the Source engine or if you'd have to stick to something like cubemaps.

Allowing vertex colors to be included with OBJ imports would be particularly useful to have since it'd be a good way to have baked-in lighting for OBJ maps. I know that there are a ton of older games like Spyro and Timesplitters that make use of vertex colors to fake lighting, and I imagine people would want to port levels from those sort of games while keeping the lighting effects. It might be a good idea to include some different blending modes for the vertex colors like Multiply and Additive in case someone really wanted experiment with that.