I love this app as it is, but these comments are coming from the point of view that for me, using traditional oils etc is always a much more pleasureable experience, so the reasons to use digital must be compelling. I think that means quicker, more controllable and more flexible for tweaking. We do have a lot of that already, undos, layers and erasers etc, but here are some thoughts on what would improve production even more for me:

My number 1 wish is to be able to turn the oil brush load setting to zero so I can use it as a dry brush blender. This is how I work with traditional oils and it is the best way I have found to produce smooth gradients traditionally. It currently does do that a bit with impasto paint already on the canvas, but because this is digital I think the blending effect could be exaggerated a bit for ease of use. I don't like the knife for blending because its shape is always too rigid, the same reason I don't like pallet knives in real life I guess.
Related to this is the fact that I would really like to see more tablet pressure sensitivity for the oil brush. Even when fully loaded in real life a lighter stroke means thinner lighter lines and it is currently a more controllable and predictable process in reality than it is with ArtRage. I also use Painter Essentials 4 and its brush tracking really helps fine tune that behaviour according to what style one is after.

-Lasso selection tool: So that I can transform and colour correct only the selected part of a layer. The centre of that transform would also be set at the centre of the selection which would be much easier than the current implementation which seems to be the centre of the canvas - not great if your layer pixels are all at an edge and you want to rotate the pixels around their own centre for eg. This one definitely goes under the digital advantages banner.

-Colour correction: Hue/Saturation and brightness/contrast. Just because people insist on tight deadlines then change their mind at the last minute Colour correction would save the whole export/import as photoshop file thing. Another digital advantage.

-Not limiting the brush/tool size to 100. Because I always work at at least A3 for print, 100 is too small for everything but last phase detailing and I find that I'm continually having to double click tool size to get a range of brush sizes more in proportion with print sizes. Photoshop's right click context menu brush size slider really helps workflow but not limiting the brush size pallet would also be really useful. Painter only has increase/decrease options in its context menu which are less useful.

-Faster move/scale/rotate layer content. Again at A3 the ability to transform a layer's contents becomes pretty much unusable.

-Faster layer processing - improving but still takes a while to do stuff like toggle layer visibility etc.

I'd love a sennelier oil pastel (soft and buttery, picks up underneath colour and creates some impasto texture when laid on thick)

I think I may have mentioned some of this before and I may add others but no harm in emphasizing These are the main things I'd love to see in ArtRage 3 anyway.