There could be three "crafting" tiers one for each class. Forging for the Titans would match their medieval / knightly type class. It would begin with improving armor but end with them creating ammunition with new and unique properties. Alchemy for the Warlocks. It seems to be a natural fit for the class. Infusing armor and weapons alike with their elemental mastery. A helmet, for example, that makes the wearer highly resistant to critical damage. Scavenging for Hunters...Taking items from their fallen enemies and make them into something better and more useful. I mean how awesome would a hunter look wearing a cloak crafted from hides of the Hive they've taken down. Or a rocket launcher created with a Vex core that can track an enemy with deadly accuracy.