Play with the reflection in right lens of sunglasses, you could add it more defined highlight to work in favor of character. It is the focal point of icon. You can also try how will dark outline between mouth and teeth work.

What image editing tool you used?

 

Currently working on another icon but I'll look into making these changes when I finish the one I'm working on.

I edit the original screenshot of icons in Gimp by resizing and indexing to generate blobs, after that I use MS Paint to polish and pixel over the blobs I generated in Gimp. Once the polished pixeled version of the picture is made I add the blue border that the original Warcraft 3 icons had and I mess around reducing colors to 255 since I believe the original icons were color reduced. I don't necessarily keep the color reduced images (to 255 colors) as I might not like them, so some of my icons have more some less colors.

 

Make use of possibility to have various layers in gimp. Also, pixel editing can be performed in gimp as well, setting airbrush to hardness 100, size 1 and flow close to 100 will do.

For color intensity, well some in-game icons feature less contrast, those are usually the drawn ones. You can edit colors by gimp tool hue saturation and reduce or increase highlight tones by curves.

 

There's a lot of things in Gimp that are annoying for pixeling.. There's not much I can tell right away but for example the selection tool is different than in paint, in paint I'm a lot more accurate when selecting pixels than in Gimp, which in some situations can be quite a nusiance.
I don't play much with the features in Gimp but I do hue edits (been doing that mostly just on the blue borders I add), brightness edits to make the darker inner edges of an icon and I increased sharpness on my Shadowhunter Lujka icon. Haven't tried the curve tools for highlights, I'll have to look into that.

I was trying out Krita before (posted some Krita sketches on here in past), it's very similar to Paint in terms of hotkeys and appeal, so it's easier to make the Paint to Krita switch than Paint to Gimp, but I generally just stick with Paint.. Especially on these kind of icon edits where I don't see a need for layers.

 

I went ahead and exaggerated highlights on both of the lenses, and I added a outline to the mouth. I also updated the resource with the new version.
Here's the comparison picture

 

Very impressive!
Thanks for sharing!!