Egregore-infested Leviathan would go crazy. Not to armchair developer, but the worldspaces are clearly up-to-date with the engine changes since they were in Haunted, they can clearly bring back old bosses (seen with the Ceremonial Bathers and ghalr... Ghalran), and the Standards mechanic was reused for Duality, and the content from Haunted is still in game since the bossfights were reused for Guardian Games of all things.

Surely the reason why Bungie refuses to bring it back is purely to spite me.