The only thing I could think to recommend is have more subtle shade transitions. The ones you have are mostly sharp and very noticeable. Sometimes it's not so nice to have that, sometimes if is. It works for having the sharp transitions like on these, but some people like them more subtle too. Personally, I like them to be subtle, but these are really good heads. Good work and my favorite is the last one, because anime. Hahahha.
Another thing, really good work on the eyes, nothing I could see to change in them. I'm not that great at art, but I try to be constructive thinker.