Lots of interesting techniques have recently surfaced that rely on encoding values into places that were never meant to hold them.
Encoding normals into textures (bump-mapping)
Encoding normals into texcoords (dp env-mapping)
Encoding distances into the alpha-buffer (shadow-buffering)
Here’s my wish-list:
Some general-purpose interpolators that can be used as operands for blend-func, texenv and texgen.
An arbitrary-precision general-purpose buffer (like an alpha-buffer but without the implied purpose)
More texenv functionality. Possibly something similar to EXT_texture_env_combine, but with the general-purpose interpolators available as arguments and without the clamping
State-objects for all this to simplify management.