I would love to see something like what Tandent Lightbrush supposedly does (see this article) implemented inside of Affinity Photo.
It is basically like frequency separation, except that it separates illumination and surface detail instead. It could be implemented in conjunction with a blend mode that would re-combine these two passes, just like Linear Light re-combines the high-pass layer with the low-pass one for frequency separation.
I don't know if the algorithm is described in a Siggraph paper or something like that, but I believe there have been several published approaches to estimating illumination and so on.
I was fully expecting for something to show up in Photoshop soon after because it would be spectacularly useful for photo editing, but so far Adobe hasn't done anything in that direction yet.
I'm not even sure if the original product is still around.