Each week have Xur sell a quest called “Gift from the Nine”. The quests are similar to the invitations, but these are purely for the collectors. Completing this quest each week will unlock an item from any past season that was not earned. Since it only unlocks in the collection any guardians that want to use it would need to reacquire it for whatever it would cost all guardians. Everything from shaders, armor, weapons, ships, sparrows, ornaments, etc.