I've studied your comic a bit, and thought about it, and I think that 2 additions to your repertoire would help the most. I think these two areas are line weight and atmospheric effect.
In regards to the topic at hand, I'll go over atmospheric effect first, as I think that's most pertinent to the discussion at hand. Simply put, things farther away are lighter than those that are close to you. I know this seems weird at first thought, but try to think of it on a grand scale rather than in the box of a small room. Mountains far away will look lighter than the rocks closest to you. Your backgrounds, because they're sharp and the color is consistent across the background, mid-ground and foreground, look a little flat. I know you can feel it, because, particularly in the ep 200 special you used effects to limit the boxiness and immediacy of the surroundings of each character.
It's good that you understand that each technique (flat vs. softened background) has value, as one can use this to create a milieu when needed to add to the reader's experience.
The foreground, in particular, is where things seem flat when rendered in the same value scheme as they are in the mid-ground. This is what stands out to me the most. Think of it as a color key printed on a piece of paper for each character, as this is what helps me. The closer each character is to the camera, the darker, more lush, a color will be. This can be tricky, in particular with skin tones as too much will muddy them, and I've noticed that you tend to shy away from over-lighting characters, as in noir art pieces. Though, you can use differences in lighting to add depth to your character expressions.
And while we're discussing foreground, it's a good time to discuss line weight. A thicker contour line will draw a character to the front of a reader's attention. For a long time I tended to shy away from using this technique, and I can see you doing the same. While for me it was reactance to tattoo art culture (I don't like tattoo art culture) it would be a detriment to you to ignore what is a useful tool that can add dimension, mass, and immediacy to your art.
I can tell from the way that you do your stepped shading that you understand light fairly well. Consider line weight as adding another tool to your light toolbox. By thickening your line in your last value areas, you've brought that edge to the fore a bit more. This is a good thing, but thickening it even more, you'll add weight and resonance (I can't think of a better word) to each character, as if saying, this character exists here, in this space. This page from MaggieW's Pandemonium is an excellent example of how to balance line weight with being representational. I'd use one of mine, but I'm nowhere near that good.
You have excellent potential, and an amazing ethic. I like your series, and though you have some new ideas to integrate, I'd really rather you not change it too much, or at least too quickly. Just slowly experiment, and I think you'll keep the same charm it has already.
