Problem is, there is a high probability that there won't be a fourth class, each guardian has their own set of skills that revolve around the three different damage types: Solar/fire, Arc/lightning, and Void damage. Warlock starts out with void damage tree but has solar damage tree too, hunter starts with solar but has arc tree as well, and titan starts with arc but has void tree as well, these three damage types and rotation of the subclasses are just too well balanced to implement a fourth type of damage or guardian. Sorry, I really love your idea for lore's sake. :c