Love Artrage been using free for a while and upgraded to Full.
It so desperately needs transparency. It would aid with watercolor effects and blending which I need for sketching.
Another suggestion on this page is related. Being able to dilute paint from the water glass. That is pure watercolor technique. I wonder if it could be achieved by some kind of selective transparency.
The degree of blending in oil color (I prefer to think of it as acrylic BTW ) is brilliant. The nearest thing to a watercolor brush is actually the wet felt pen which is great due to its tranparency which means blue over yellow makes green. Great. But the degree of blending is still not that delicious running together of colors of true watercolor. and not to the extent of the oil/acrylic paint blending tool.
Spatter is another watercolor technique that would be both desireable. and popular.
Now, you have the opaque paint media and drawing media brilliantly imitated, those of us who like tranprent wet media are not so well off. Transparency please. Perhaps a slider. like the softness etc dials.