I just had an idea going off of your post that seems much more like something bungie would do. The hunters learn new subclasses from the fallen, the warlocks from the vex/hive (hive magic bruh), and the titans from the cabal. This seems like something bungie would do more so then add whole new classes because people would already have tons of hours put into the characters that they have and you could get them through quests with the associated races.