I think one of the reasons helms often look bad in many games (though DA seems to go out of its way to make most of them hideous) is due to hair. Many helms should show hair but in most cases if hair is seen to any degree, your character suddenly has super short hair that's flat against your head. To do it "properly" there would have to be a helm model for every hair style to account for that style. Basically its taking a short cut to do it the "show no hair" or "wearing this hat shaves your head" thing.
Then you have the idea of trying to use real world inspiration for helms that often get lost in translation due to how character models fit together or because, to our styles today, some of that old timey wimey real world stuff looks absolutely ridiculous. There's a certain aesthetic to good looking fantasy armor that is often left behind when it comes to helms/headpieces. Vivienne's default headress actually looks good IMO. It "fits" her and that's the only helm I'd want enabled if I could cherry pick. I'd want all her helms to look like that if possible 
What I'd like to see game designers try to do is making the models for their armor/helms a bit less bulky. Take that upside down bowl on Viv's head in the screenies above. It's so thick! I mean I know it's supposed to provide protection but, it juts out noticeably from the sides of her head. Helms that aren't metal should also have some slight movement to them when the character's head moves such as cloth and leather. I'd also like to see more circlets and highbacked collars as head options to balance out the deluge of hair/face hiding helms that typically dominate fantasy games. Caster's in particular make more sense wearing a circlet than some bulky helm.
I'd also like to have helm visibility settings per character in addition to just 1 on/off global setting. In the case of Vivienne and Blackwall I'd like to keep their hats on... while other characters have it hidden.