I think that there should be more incidental big bads
Your characters go through a door and find a boss arena with a lich in the center cackling to himself. “Finally! I have collected the seven keys to the Apocalypse Throne! Soon, the prophecy will be complete and a thousand years of blood shall descend upon the world! Mwahahaha!”
No-one’s ever mentioned an “apocalypse throne” or its keys or a prophecy about a thousand years of blood all campaign, and you have no idea who this guy is. You stop him and the game just goes on. None of this ever comes up again.