If you please, I would hope the MegaDungeon will have the same degree of relevance and revelation to Project: Eternity as did the Modron Maze in Planescape: Torment.
Give meaning to the Dungeons existence! Alongside the obvious loot drops let the discovery of rooms and passageways lead to interesting revelations about the game's lore and previous visitors to the dungeon levels and its current inhabitants. Make the connection to the upper world surprising and unexpected! (Specific comparisons left out as to not spoil even one small bit of the Modron Maze.)