I think it would be really great to have a few brushes that mimic hair textures, whether it be strands, braids, dread locks, etc. Even if it’s just basic shaping that we then add strands over manually. I love doing hair, but manually drawing every strand along each curve or every curl shape manually when I could benefit from a multi-strand shaped brush can sometimes take me hours and then it takes way longer to do just that one small piece of the whole painting.
The same goes for Liquid textured brushes. Brushes that mimic drops of liquid that could be blended out into splashes or drips, or ripple texture brushes to mimic disturbed water surfaces. Again, things that can be done manually but would benefit from a brush.
And finally, since we have grass brushes, feather brushes, and fur brushes, what about leaf or petal shapes? It can be used to create quick bloom heads for flower fields and clusters of bouquets and the like. Even if it were individual petals to make simple flowers like daisies or layered to make roses.