Simply put, the three destiny classes are based off of the classical fantasy classes. Those being The Knight, The Thief, and The Mage. Titan, Hunter, and Warlock respectively. The classical classes are based off of the three most basic ideals/ concepts/virtues. Might, Cunning, and Wisdom. To truly create a new 4th class would be to create a new concept that is completely and wholly different than the original three. Any combination of the current three, and you just get subclasses. Exactly like how colors work. There's only 3 original colors, red, green, and blue. Each fantasy class being associated with a color, Might is red, Cunning is green, and Wisdom is blue. You can see this in almost any fantasy setting game like skyrim and similar. Making a 4th class would either have to be something completely new on a level such as a new base color, or it would have to be a mix of all 3 classes. A jack of all trades class. Which would be ridiculous because from a gameplay perspective, all three destiny classes can each cover a specific role by themselves. All 3 can do crowd control, high single target damage, and general DPS. There is no need for a 4th class as every class in the game has a subclass to fit a specific role that complements the other classes and their subclass quite effectively.