Like someone else suggested, if we are to get a 4th class, it'd be cool if it were some kind of anti-hero-like guardian. Not even a guardian, but some kind of ally of the Vanguard. Someone who isn't too fond of meddling in the Vanguard's affairs, but will help if it benefits them and his/her clan. Druid probably suits this description, since they typically use some form of dark magic/trickery.