First off brilliant program guys.... bloody good value for money!!
Lean mean and highly responsive with an elegant interface that almost belies the programs power.
Please disregard the following if already mentioned in previous posts...
Custom brushes feature would be nice. Maybe similar to the palette menu i.e.
Miscellaneous brush set
An example of a bush set could be...
5px detail brush
10px general brush
30px acrylic brush
Small Smooth Blender oil
Hard Brush Blender oil
Small Palette Knife
Glow Air Brush Effect
Note: Optional tie in to the canvas with it's roughness and scale etc. as it has a major impact on blending or chalk texture effects etc.
Of course I don't expect to be as good an artist as Rembrandt because I am emulating a brush style but a form of this menu might cut down on R.S.I. and I can never remember the cool effects I have come up with when coming back to a painting.