Warframe
Their assassin's mark system.
In the game (Warframe), when you kill a boss, you get an assassin's mark pertaining to what faction the boss is from. Grineer, you get the Gustrag Three, Corpus you get the Zanuka Hunter, and generally all boss will somehow trigger Stalker.
So after you get the mark, there is a 2% chance that any of those assassins would appear in your next mission. They're hard at lower levels but very easy at higher levels (except for the current Shadow Stalker).
I hope FFXIV would adapt this system and add a certain chance that a rare enemy would join the dungeon or trial. The game lacks variety when it comes to randomness and I think this would be a great addition.
I would love to see Odin suddenly appear in Amdapor City HM just before the final boss, or when you were surprised by an S rank mark in a dungeon. Or even to see a new kind of enemy right after we kill the last boss.