Deduplicate files in fallback image
This just occurred to me: Currently, the fallback image contains files that are also in the regular image. By using the kernel's support for catting initrds together mkinitcpio could reduce the size of the fallback image.
Any files that would normally be dropped by the autodetect hook would go into a slim-fallback image instead. Then a fallback bootloader entry would reference the regular and slim-fallback images and get the same effect as a if it only used the old fat fallback image.
This would allow reducing boot partition space pressure without having to forego fallback image creation altogether.