I agree with everyone that's said ponytails and pigtails.
My other suggestions:
-Being able to edit a particular hairstyle's length. In other words a "long, straight" hairstyle could be made to be just past the shoulders are all the way down past the butt.
-Being able to "mix and match" hairstyles. Like, being able to add a small braid to an existing hairstyle or being able to add bangs, etc. to an existing hairstyle.
My thinking on these first 2 suggestions is that it could save you time 'cuz you wouldnt necessarily need to create so many new hairstyles and people could play around with existing ones and create what they want. Could maybe just be added as an option into the HairStyler add-on?
-Headbands, hairclips, etc.
-African hairstyles e.g. afros, dreads, etc.
I could probably think of more if I sat here long enough, but the fiance is gonna be done with work soon and I need to go do some cooking. Keep up the great work Jak!