What about one on silhouettes and shading? As in parts of clothing or body parts etc that would have be a bit darker, kinda. Kinda like how in manga they use screen tones for things like that. Pretty much how know what parts to shade.
Another one may be how to draw or add highlights to hair or show like where the sun hits. Like when you have a character with black hair i don't think you'll ever really see their hair being fully black. They'll have like this white part to show where the hair and stuff are going. I don't really understand that one so it may be helpful for some others as well as me.