Imagine 1e modules that were never solved/adventured.
White Plume Eruption: The Archmage Keraptis, astride Dragotha the undead dragon, with his legion of monstrous minions, several of whom are armed with artifact-caliber weapons have begun their conquest to take over the world.
Against the Giants: ...is, literally, against the giants. They got their act together. No one took out their leaders one by one and now ACTUAL ARMIES of giants are beginning a coordinated conquest of the mortal realms.
March of Horrors: Acererak the demi-lich got bored with expanding its consciousness through the multiverse and returned to the mortal plane. The "jeweled skull" is floating through the world, gathering all evil creatures in its wake of horrifying unstoppable magical power: taking on the abilities and powers of all of the souls its eaten over its ages. Limitless spells of all types. Proficiency with all types of weapons (mastery with several), [anti]Paladinic powers. Druidic shapechanging to give itself mortal-seeming forms (still with jeweled eyes and teeth, in any shape, naturally

.
The Caverns Found: Iggwilv's vampire anti-paladin daughter was awakened, defeated those who disturbed her rest and has vacated Tsojcanth. With an army of unique monsters, the evil humanoids from the surrounding mountains, and a small horde of indentured demons, Drelzna is on the move to return her mother from the land of the dead and bring/offer her father the Material Plane to call his own.
And so on. Just take the standard AD&D (or D&D) modules and carry them to their logical outcomes if no "heroes" showed up (or succeeded) in time.