I like the skull man and I felt as he was being neglected, so I did an edit myself.
I guess most of the things I have changed (without mentioning the usual stuff: hue shifting, AA) regard the actual assemblage of the character. I've tried to do it in a manner that would preserve the overall feeling to your original sprite.
First, I basically tried to change stuff that would make the head look more like an actual skull. The nose was brought up and the cheek bones are now more prominent. I've also tried to blend the teeth to the rest of his head, bringing the upper jaw out of his face, mainly because they were showing as vertical bright lines on an area that (according to the original anatomy) should be shaded. Additionally, I choose to give him a more angry look (with frowned eyebrows and squinted eyes) because that's how I figured this character behavior is supposed to be as he's leaning forward with his arms back and closed fists. Basically a defying pose. But I assume that this last bit is just a personal interpretation of how you drew it.
Another important thing worth mentioning: As the head has too much small details (and is his most distinguishable feature), I though it would look more readable if the rest of his body would just have a darker an less saturated color, using a dark shading "frame" around a more contrasted and saturated face, bringing all the attention to it.
I did add two more colors but I think I've put them to good use.
-Stefano
PS. I hope I didn't mess the sentences beyond understandability. This is as better as it gets...