Is not a very common idea, but you could make a helmet that works as a hairstyle, you just need to position it in the right place and you can have basically a wig.
Also as any other helmet, it could use different textures so you would have an ample variety, and as each race has a separate model for helmet you could do some variations according to the gender or race.
Of course this would prevent you from having a circlet helmet and the hair at the same time, but personally I can't see that as a real problem.
