How much of a stretch is it to believe that SE could implement this into the World, seamlessly.

Kind of how Chimera stands waiting for those with the Quest active, but does not appear if you do not.

The person who has the Set of Rare/Unique Items in Inventory, or Traded into a NPC would be able to initiate the fight(most likely instanced).

Items could be obtained Via FATEs, Dungeons, Trials or DoL.

Would be pretty cool I think!