I'm finding it very hard to get perfect field flatness even with flatfield correctors - resulting in somewhat deformed stars in the outer zones of my images.
I know there are deconvolution algorithms which can "correct" for general star deformation based on a star PSF, but all such processes I've seen either work based on a theoretical PSF (a gaussian for example) or based on the PSF of one star or the average of multiple star. They all have in common that the same PSF is used for the deconvolution across the image.
An algorithm that could analyse star shape across the image and deconvolve based on a variable PSF could be a very valuable functionality, especially in combination with mosaicking, where the variable correction might allow more of the edge of some images to be used.
Certainly not the easiest of things to implement, but as far as I can tell, no other astro-software provides such a thing at this time - as long as the faults are relatively limited, it could be useful both for correcting field flatness and rotational unsharpness.