Heightmap-based normals are great, but the one limitation I typically find with them is that you can't easily do any normal variation smaller than 3 pixels wide. For doing seams or grooves at a fine detail, you sometimes want two opposed normal pixels in a V shape side-by-side like this:

To do this right now I need to export the UB Painter normals and draw over them manually myself. It would be nice if there was a way to do this inside UB Painter, either by being able to somehow set the height at the "edge" between two pixels (maybe some tricks with the RGBA channels on the heightmap?) or by being able to nondestructively override the heightmap in certain areas to draw your own normals.
This one is probably a lot of work for not all that much gain/importance, but it would be nice to have in some specific situations!