I really love what you have done with ArtRage, especially the very intuitive and tablet-friendly interface (since I use a tablet pc, I don't usually have access to the keyboard when painting) and the open canvas that allows me to fully engage with the art.
I had started with a whole long list, but really, when I thought about it, there's really just two that would make life that much more rawkin'. After all, if I wanted complex painting program, I'd use Painter. If I wanted a complex image manipulation program, I'd use Photoshop. What I love about ArtRage is the combination of simplicity and power. Hopefully these two suggestions enhance that.
1) Lasso/Marquee that allows resize/reposition.
I know that doesn't fit very well with the "painter environment" feeling, but one of the benefits of digital painting is being able to do some last-minute tweaking of position/size. It's frustrating to have to export it to Photoshop, move, then re-import...
2) Inking/Calligraphy pen
Setting the crayon to ultra-soft or setting the brush to instant-dry is not the same. Sometimes one just needs beautiful solid ink lines. And "Pen" seems more to be more marker-like than pen-like.
Perhaps ArtRage already does these two, and I just can't find it. Apologies if it has already been suggested.
Thanks for the awesome program!