While the markers on Art Rage are lovely, one thing I think would be really nice would be a watercolour marker kind of tool. This tool would only be able to darken things, like if you work on top of things with a water based marker (The current marker tool seems to behave like spirit based markers, so won't get darker than a certain point.) This would be really nice for building up shadows or base drawings, like doing an ink wash. A tool like that is one of the few things I feel is really missing from Artrage to make it more like real-media, as I've always been more of a transparent medium user than opaque.

Another, more 'fun' tool might be a splatter brush, that could be used to make chalk dust or paint splatters. The latter would be expecially fun, even more so if you can adjust the size of the individual splasts, and the angle, so you could get mad action painting effects like when you drop paint in splashes onto a canvas, or slash as it so you have sprays of paint splats.

On a similar note (though it would probably be very hard to do this and not make it horribly slow), a 'super wet' brush could be cool, the kind that would make the paint actually run down the canvas as in real life you can get some lovely textures with extra runny paint.