We have the 3 main class archetypes found in almost every fantasy game (mage/thief/warrior = intelligence/skill/strength = warlock/hunter/titan) Not sure how to make a new class that doesn't already fit into one of these archetypes, for example summoner could be thought of as a subtype of warlock. All of the classes have elements or options that make them good supportively, defensively, and offensively.

The only archetype that I can think of that wouldn't necessarily fit in with just one class would be some sort of "faith-based" Disciple class or something. Maybe for a future expansion, some guardians betray the Traveler for power. Too injured to seek a new race to uplift, the Traveler has to make do with the guardians still faithful to it, and grants the faithful ones additional power as well. Due to these events there is a new class, with subclasses that signify alignment with Sky (faithful option, gives us abilities similar to the race(s) that created the Traveler), Deep (betrayal option, includes Hive or Taken abilities), or some sort of neutral philosophy (Vex-ish or fallen-ish?), unlocked by your choices in the story (which can be replayed to unlock the others.)