WIP
The only one artist who was making a good heads like that was Membrain and only.
He was making a heads which could probably been placed on the real canvas.
Your lines are dirty pretty much, you know, when you make a sketch you make these lines to eraise them later. Color pallete is broken, these colors don't go together at all.
You are trying to make it bulgy but it's still too flat just because you don't know how to use lights and shadows. Snake mapping is fucked up I see it already, it will be broken.
Man, seriously, untill you are not that good in painting try to improve yourself in a regular art, learn 2 academic painting, plastic anatomy etc. if you are so interestend in stuff like that. Untill that you will not make any worthy.
I'm not trying to offend you, I'm just making you tips don't waste your time on what you can't do. You are asking "What can I do with an empty parts?" but empty parts are not you real problem and also if you don't know what you want to do with your texture - don't do it. Do what you really like to do.
You can ignore my tips, do as you wish, just at that point it looks bad.