Many DMs recommend using the Dungeon of the Mad Mage Companion from DMsGuild, which provides a structured "Game Show" script for the entire campaign.
Have another group of adventurers (the "fan favorites") descending at the same time. This creates a race for loot and a sense of a living world.
Halaster has grown bored over the millennia and has invited "contestants" from across the planes.
Chasing a legendary criminal who fled into the Yawning Portal.