ArtRage has ability to simulate most of these techniques, but take a look at any of these Bob Ross videos on YouTube:
It would be cool to have some of these as more dedicated tools / effects:
Liquid White Canvas Option
Affects how paint is accepted on the canvas
Proper behaviour for brushes with no paint loaded, for blending, fluffing up clouds etc.
Fan brush, round brush, liner (rigger) etc. so you don't have to spend too much time constantly changing your brush pressure, loading and thinner etc.
Maybe each brush type also remembers it's colour too, as if you had several virtual brushes by your side.
Maybe not quite a big as Bob's! But would be useful nonetheless!
Loaded Palette Knife
Ability to have a roll of paint loaded on the palette knife for scraping colour on top of the canvas
I believe any of these tools would be in-keeping with ArtRage's philosophy - intuitive, natural painting, without being a bloated package like Painter!