I've been looking at some development of a very impressive auto-fill filter for lineart on G'mic and thought it would be something the Affinity devs might be interested in. Basically, it analyses artwork with an algorithm to determine contours and shape closures which can then be filled as a base for painting. It speeds up a monotonous colour blocking step of painting.
Not a priority feature at the moment, but I thought it would be an amazing addition for illustrators working with raster imagery. I'm sure affinity could simplify the tool even more for usability. I've posted two videos that demonstrate how it works.
https://www.youtube.com/watch?v=t-3aBQeuBc4
https://www.youtube.com/watch?v=Rl76PJZTuio