I would like to see an OPs where you only fight one boss, but in multiple stages. I envision this to taking down the Shroud in one of his lairs. There could be a few stages of tank and spank, a couple of puzzle stages, while continually moving through a highly booby trapped lair. Each time you finishes a phase on the boss, a chest in the room will unlock for loot (Think how the Jarg and Sorno, Infernal Council, etc. bosses worked for loot.