Started as a comment by GirlMini on Alatariels great Female Minifigure project, here is the implementation of that idea:
Two-sided faces - one showing a male and the other one showing a female face.
I used the following example figures: Police officer, plumber and photographer.
You have to take care of the hair yourself. Not all kinds of hair works for these figures.
Another shot without hair showing that the back-side of the face is the male one.