The warmind cell mods, season of the lost, charged with light, etc. You do have to purchase them right now when they become available (for 10k glimmer which isn't expensive in the slightest for me, but perhaps for a new player). But if they're so bent on making them hard to get, why not give you one you don't own yet after a rank reset or add them all to a kiosk where you can purchase them for glimmer and enhancement cores/legendary fragments?