How about an Exotic Artifact per character which grants the ability to summon a powerful enemy to help you fight for a 40 second time period ( an Ogre, Taniks Kell, Cabal Warlord).To use the artifact there is a 5-7 minute cool down, your character must have full super energy, full grenade, and melee energy. Also it will use %50 or your health to activate it.