Definitely like to see a fourth subclass that utilizes the green Hive light, or Dark Light, as I like to call it. Uses Poison, Life Steal, Energy Drain type effects, etc. At some point, Eris offers a quest to learn forbidden knowledge of the Hive. As far as full-blown classes, I'd go for the standard cross-classes similar to Mass Effect: Hunter/Titan, Hunter/Warlock, and so on.