Is there any specific reason you have for believing that there's a fixed-function part in the GPU? I'd be interested to know the evidence/thought process that lead to it (if it's not based on confidential info).
The one potential fixed-function feature that sticks out in my mind would be lighting. Good lighting is good lighting whatever game it's in, so fixed function lighting hardware is a sensible area to trade off versatility for efficiency. One very interesting (although also
very unlikely) possibility would be to include dedicated ray-tracing hardware in the GPU. Now, I'm not saying that such hardware would produce actual fully ray-traced rendering in games (we're still a good few years away from seeing that in games), but even rather limited ray-tracing hardware could be used to augment standard rasterized rendering.
For an example, have a look at
this video. In a nutshell, the technique they're using is to render direct illumination in the normal manner on the GPU, and then to use the CPU to perform photon tracing for one or more bounces, the results of which are then rasterized into a lightmap for final rendering. It can produce pretty nice effects in real-time, but it's running full-tilt on two quad-core CPUs to do so, and even then doesn't manage particularly good frame-rates.
CPUs and GPUs are pretty poor at handling ray-tracing, though, and there is dedicated ray-tracing hardware out there, such as
this academic design and
a ray-tracing co-processor from Caustic Graphics, who are now owned by Imagination Technologies (who make the PowerVR GPUs used in phones like the iPhone). Most of these technologies (and there are quite a few others, not to mention papers on the use of FPGAs for the purpose) are intended for full ray-traced rendering, though, calculating tens of millions of rays per second. A ray-processing unit on a standard GPU would only need to be able to calculate a few hundred thousand rays per second to match the performance of the video above, and perhaps a million or so to do so at a good framerate.
I don't know if this is technically feasible (blu will probably arrive shortly to tell me the many reasons it isn't), but it's interesting to speculate what might be coming down the tracks in terms of new GPU hardware.