well overall I think it just looks too flat, whether it was the type of style you were aiming for I'd love to see this with more shape.. I did a quick edit.. Ive messed around with the pallete slightly and added 1 more shade + another to AA the white outline, but its really making the most of your palette.
adding Lighter shades tend to raise parts and darker pushes it away especially with a black and white palette like this, its much easier to just concentrate on contrast as theres no Hue/saturation changes.
The structure of this is pretty solid, its a lovely style to work with, but don't restrict yourself with anything too blocky unless its on purpose.. not really sure about the arms, they seem to finish too high imo and whether there are hands is questionable from 100%
heres it with a colour suggestion
goodluck