In D&D, the concept of the typical dungeon is a well-known one. A labyrinth of five-foot corridors and perfectly square rooms, full of elaborate traps, wandering monsters, and hidden loot. It's a trope dating back to the original Chainmail game, and iconically tied to the entire genre.
There's just one small problem: how unrealistic they are. Very few people would be willing and able to create places like these. For every mad mage creating a nonsensical maze, there are plenty of combat-full goblin hovels and orcish warcamps that don't make sense as a traditional dungeon. My idea does something interesting with this concept, and turns it into an entire plot line for a campaign.

May I introduce...
The Dungeon Fungus

The "dungeon fungus" is a mysterious infestation of something resembling mold. It infects locations - typically enclosed spaces, like caves and large buildings - and magically twists the layout of an infected area into something resembling a stereotypical old-school dungeon. A fungal den grows traps and monsters to defend the infected area, created out of the actual fungus. Materials and creatures created by the dungeon fungus outwardly look normal, but upon their destruction, they crumble to reveal their true form.
Each fungal den has a core, typically a large object or particularly powerful fungal monster. If the core of a particular infection is destroyed, that infection (and any other infections spawned by that one, who haven't yet grown a core of their own) are destroyed. Fungal creatures instantly die, and over several hours, the layout returns to normal.
The growth of new dens goes in a several-step process. First, fungal creatures from another den assault an area, driving out any previous inhabitants to clear the way for new growth. Second, these creatures begin to spread the fungus in the new territory, which begins to morph the layout. Third, with the accumulation of fungal growth, a new core is spawned in that area. After that, the core and the den grow out more inhabitants and traps to fill the area. These fungal monsters often mimic the area's former inhabitants, but usually with some changes, and sometimes as completely new beasts. With this new den complete, monsters are sent out to attack another area and create a new den.
Sometimes, inhabitants and treasure remain in the original area after the fungal attack, and are incorporated into the new design. Infected creatures are under the control of the fungus, but they die as normal, and return to their former state when the core is destroyed. Treasure can still be recovered and brought back to civilization.

In The World
The dungeon fungus is a plot hook big enough to serve as a seed for an entire campaign. Adventurers could spend their time destroying cores to fight back the fungal influence, protecting borderland civilizations from attempts to create new dens, or seeking into infected territories to destroy the source of it all.
Many smaller events could arise from the fungal infection as well. Normally evil creatures like orcs and goblins, once foes of civilization, might be persuaded to work with heroes and fight against the dungeon fungus. Major calamities could purge vast areas of infection, changing the course of a war against the fungus.
In my idea of what this would look like in a world, the dungeon fungus may have overtaken less civilized areas of the world, beginning to encroach on frontier cities. However, it could easily be at a different point in its growth, to allow for a different sort of campaign. Perhaps heroes defend last bastions of civilization against a world overwhelmed by the dungeon fungus, or perhaps it only just begins to grow in the most remote corners of the world.

So, thoughts or questions? I think it's a neat concept, but there are probably issues I haven't thought about. Also, if you want to use this in your own campaign or setting, feel free.