I'm imagining one way of dealing with sensitivity to motion in the periphery, when doing foveated rendering, could be to do the rasterisation step at lens-matched, but not foveated resolution; Then, every frame, build a new mask from the resulting image, favouring edges (this should take nothing more complicated than a pair of more or less standard convolve and dithering filters); The mask then determines which pixels get to get shaded, and intermediates would be interpolated. Whether there would end up being any savings, once everything is added up, I am not qualified to say, but... :7
The big problem is one of standards, variability in features and capabilities, and whether technology and methods will remain relevant for any length of time...
There needs to be one interface that lets the engine talk to all eye trackers, so that developers do not have to chase after ever more new products and implement access to proprietary APIs for each and every one. Fortunately this could be assumed to be covered by whatever comes out of the OpenXR working group.
If, say Unirealbite engine adds support for something, such as a new way of doing foveated rendering for a certain headset, developers who have games that use this engine will at minimum have to make a new build of their project, using that latest version of the engine. As far as I know, switching engine versions is "a bit" of a headache for developers, as it is -- there is always something that breaks. :7
In todays headsets, the lens compresses and stretches parts of the image, and the image is always on a rectangular flat plane; These matters, which a lot of worksaving rendering techniques can be built around, could well become irrelevant with future curved lightfield displays - you'd at the very least not want to "hardcode" yourself to such constraints. Future rendering methodology could well drop rasterisation altogether and take the plunge into raytracing territory (...if I have my will, anyway :P) -- That would invalidate many optimisation schemes, and open up for many others.
It's hard to be forward thinking, and ever get anything done. :7