I played a game called Tera Online for a little while. It was very similar to this game mechanic wise. They had mobs that were known as BMs which were massive versions of normal mobs and they had 2x-3x HP and had some different fight mechanics which made them hard to solo (you'd need help from 2-3 others). Those were fun, but the downside is they put them into quests (ie. kill this many of such and such) which was annoying since hardly anyone played that game. If they implemented monsters that were to drop BiS loot then they'd need to follow some similar mechanic. The new EM primals are out that do just that, however, I do remember what you are talking about and I do miss it. Unfortunately the way the game is I don't see it being feasible to implement such mobs. What would be the point with the primals already in the mix?