I couldn't help but notice some inconsistencies with the light source on this one, so I shifted around some colours to get that more inline. Most notably was the hair which just seemed to have the sun coming from a totally different angle. I also highlighted both legs and hands the same way just to see what would happen really. Think it decreases readability, but hey now I know.
You also had a lot going on on that ball, so I cleaned that up.
Also for the rim, given it's a flat surface and not curved (unless it is curved upwards, as your shading suggests), you can just make it one solid colour.
I've noticed people tend to shy away from having a large area a single colour, but sometimes it's what has to happen. The rim does look a little dark to me now, but it was just for illustration's sake.
I haven't made any glass in pixel art yet, surprisingly, so I don't know how you'll make it look like a glass orb, but I attempted it any way.

Those are all the ideas I have. Very cute witch, good work, carry on.
EDIT: Well I got carried away. Quite an inspiring little piece, and having a closer look you really have chosen a lot of your lines really well. I tried to rectify some of the issues I had and then I just ended up doing more and now it looks like this.

Added a belt buckle on the hat, tried using an over saturated colour on the bottom of the crystal ball to make it look glassy, removed a lot of the black lines and started work on a palette.
I tried not to lose any of the original appeal, but that's virtually impossible.
Hopefully you find something useful out of this.