I have been playing with the smudge brush in version 4.2.8 of Krita and so far I have been impressed. The smudge engine currently has support for animated gih brushes, canvas textures, brush dab spacing, jitter, rotation, and full support for Wacom pen rotation. The only feature so far that I would like to see is the ability to control how the smudge brush blends individual color channels (RGB, and Alpha). The ability to lock how much the smudge brush blends with the alpha (transparency) channel could be useful for simulating painting with a wet oil brush on a clean, dry canvas. When there is not already any paint on the canvas, oil paint will not mix with the dry canvas and only spread out the paint. Unfortunately, I don’t have any oil paints at home so I don’t know how to demonstrate this property better.
The ability to limit how much the smudge brush blends with the other color channels (RGB) might be useful as well, but I cannot currently think of a reason for only wanting to blend certain color channels but not others.