I don't see how a Thrall could necessarily be a boss on its own without first becoming an Ogre. Thrall are the weakest creatures in the game. If there [i]was[/i] a Thrall boss, imagine it severely resistant to all weapons types except for Kinetic, and it could summon other waves of Thrall and Cursed/Hallowed Thrall. Not only that, but the Thrall Boss could move faster than the other Thrall, and if it managed to touch you, it'd insta-kill. That would make it terrifying. Other than that, I would like to see another Vex boss instead of Hydras. Maybe a Harpy, maybe a Minotaur. The worst boss imaginable is a Hobgoblin, but I would love to see Bungie create that just to see how sadistic they can truly be... haha.