This idea may have already been touted and I apologise if it has. I love painting in artrage and I love the way it makes me feel like I'm painting rather than fighting with technical interfaces, having to create brushes and so on.
But ... I would like more brush options, although I really wouldn't want it to go down the photoshop route of brush definition.
It occurs to me that it might be possible to have a 3d object in the shape of a brush that is flexible and has collision with the 2d paper plane. This way as the shape is moved around the page it bends and distorts depending on the way it's manipulated by the artist. In doing this the surface of the brush touching the page would change and create a different mark. If you were using a fine tapered brush and applied very little pressure you would get a very thin line, albeit with small discrepancies as it interacts with the paper, the more pressure you applied the more of the brush would be pressed against the paper there by giving a much broader mark.
In this way I believe it would create an even more realistic painting experience, differing 3d brushes could be created for all the real world couter parts, incorporating differing stiffness, shape and so on.
I hope I've made sense. I will try and find time to produce an animation of what I mean.
So ... what do you think?