Hair realism: better textures, some wispy movement (but not too distracting), attractive hair styles, and a nice color palette with complementary color highlights, are what I'd like to see. improved.
The real challenge seems to be getting blonde hair to look good with a proper skin tone so it doesn't look cheesy. I'm not trying to go off topic or slam brunettes, because I am a brunette. Ashley's brunette hair was well done in ME-3.
The software is getting more advanced, and other games have better hair designs, so I assume it will be worked out somehow, since so many fans are clamoring for these improvements in custom character designing.
I like a few of the brunette hair styles that fans are posting in this thread, pretty much. I'll try to find a few examples to post here that I hope will look good in-game.
Edit: I assume that having wispy hair will require a better discrete gaming graphics card. I hope it doesn't require a Geforce GTX 980 Ti.