I had never done any comic, but picture is a picture (I'm using GIMP).
Some ideas IMHO:
- You can use some "trace contour" and copy it to separate layer for thickening or strenghtening
- You can smoothen the lines by selective blur or resizing
- You can add shades with semi-transparent layers
- You can increase or decrease "magic wand" selection details and size for more accurate filling
- You can do filling with light gradient, not only solid colors
- You can add small bump or shadow to complete shapes to make them look more 3D
Good luck!