The fifteenth wish will become (or may already be) available during the Season of Opulence. It is linked to the mission, ‘The Other Side’. The wording here is important: The discovery of the wish is linked to the mission, however, may not be physically visible inside it (for example, on a plate). However, the mission, “acts as an origin and will play an essential role in its discovery”. While I am unsure of what it achieves, I am aware that it is more important than all other wishes, and will significantly further The Dreaming City storyline from Forsaken, in preparation of the events in Shadowkeep.