line modifier, faster stencil, unlimited paint load etc
Have you considered simple line hotkey (to constrain tool to a line via modifier key)? It would be immensely useful for making layout grids, setting up vanishing points, painting un-organic shapes (machines, constructions) and other such work where lots of lines are needed and it's too much work to do them by freehand or by ruler (which, sadly, is excellent but bit slow to position). Control-key would be fine key for line-modifier.
Another very useful construction tool would be round "ruler" that could be stretched in addition to scaling. Dragging corners would rotate it, dragging sides would scale it and dragging the center would place it. There would be no need for different transformation modes, making it very much faster to use. Having stretchable/scalable circle would make custom-made ellipse rulers unnecessary. This would make it so much faster to draw/paint curved surfaces and construction lines for bridges and other arcs. Stencils are great but slow to use. On similar vein a hotkey for showing/hiding all the stencils at once is badly needed.
Would it be possible to have limitless amount of paint in tool? I constantly run out paint with brush and while it's realistic it's *not* fun (so I use paint roller instead, I like long strokes). I've also noticed that tools become very slow when they are big with 2k images, so maybe there's some clever way to optimize this (bucket/scanline/tile-rendering or some such).
Would it be possible to have fractional tool sizes? 1% paint roller is too big for detail work while it's also lot more fun than paintbrush.
I'm an animator so I would like some sequential image support (no actual animation support), either by having hotkey for jumping to next/prev layer (pageup&pagedown!) or having hotkey for loading next image on directory. Both would be useful for other work too, I presume.
Would it be possible to have non-incremental strokes as an option? There are tools (like pencil) that are non-incremental in real world too. I ask this because I constantly do something that has right shape but wrong color. Instead of repaint I would just rather recolor it, and having non-incremental tools would make it a snap. And this brings me to my pet peeve, which is digital brush. It would be incredibly handy to have something that is non-incremental, can make sharp lines, doesn't run out of paint, doesn't have any texture and is totally controllable.
I don't think you need to put everything to buttons if it makes interface cluttered. For example "F5" is about the best new feature in Artrage (some indicator would be nice though to make sure lights are off). Similarly you could just leave techie stuff invisible if their UI is the problem.
Cheers, this is the best paint/sketching program there is.