I would like to see a channel mixer offered as a filter / FX layer in Affinity Photo, similar to the one in Photoshop or Resolve, except extended to support the alpha channel in the same manner as the others.
For the benefit of those not familiar with it, I've attached a screenshot of the "RGB Mixer" in Resolve. The idea is that for each color channel, you can provide a mix of the color channels in order to make that one up: in this case, the output red channel is made up of the full input red channel, the output green channel of the full input green channel, etc.
I'd like to see a set of controls for each of the "output" channels, allowing a mix of all of the "input" channels.
This has numerous applications: custom grayscale mixes, fixing bad channel data by creating a mix of the others, a form of chroma keying (by mixing into the alpha channel), etc. - it would provide a number of capabilities in one blow, and I would think it should be relatively simple to implement.