Hey I like your sketch
Edit: oh shit I just read the sketch was found on the net
We can get this looking very sexy.
IMO go for a style like seen in Owlboy and Shantae 2 Risky's Revenge.
1) Lets decide on a resolution.
IMO you should go about half what you have now (depends on character sprite sizes really) unless you have a reason to use this higher res?
2) Loosen it up. Get rid of all the black lines and just paint it up from blocks of colour.
Work rough with larger square brushes then clean it up after.
Don't analise over perfectly straight lines, instead let things be a little bit wonky and rough around the edges with delapitated textures as it will add character.
3) IMO go back to the original sketch design as it has more interesting shapes and materials.
4) Instead of being directly front on, rotate it so u see a bit of the side too (but not the top). It will look WAY better like that.
5) Pick a strong light source e.g. sun from top left and stick with it. This light source and the shadows it casts will be what builds up your forms.