I'm actually in school right now (fine arts) and learning quite a bit (not digital but the lessons translate over) not sure if you are too, but you seem like you might be... Haha. So in those regards, my profs would probably say something like... You could make it pop more with possibly a thicker line going around the outside edge of the image, and things like specific color choice and more subdued hues, complementary shading, yadda yadda. Basically what that means is that full saturation, in many cases, is not as impactful as more subdued or desaturated tones with maybe 1 or two pop colors.
In terms of brushes, I just use the standard Sai set and play with the opacity and pressure sensitivity til I get what I want (be it lining or shading) and blend with the watercolor tool. I don't usually use airbrush for anything more than wide-area shadows, because I find it kind of weak for my tastes.
oh, and idk if you already do this cause im looking at it on a cracked smartphone but doing drawings on really big canvases, like 3000x3000 px minimum and then shrinking it down seems to crisp up and increase the quality of most digital pictures, though I usually have yo sharpen it later in photoshop.