Ancient Magic has a wildly differnt source than moden magic, unknown to the current inhabitants.

A disaster melted the icecaps, covering 90% of the land, leaving scattered settlements

A ritual transforms all man-made buildings into living beings capable of slow but reliable locomotion.

Goblin gadgeteer gangsters guarding glittering golden globules.

A mystical bazaar where people gather to barter and swap their memories.

Come summer, the men rule the land; come winter, the women rule.

Dragon hearts continue to pulse with magical power after death, allowing mages to use them to power arcane devices.

A world where there are no humans, only elves.

A city ruled by priestesses of the eternal flame who tend a web of fire that links the hearth of every house to their temple at the center of the city.

Seafaring nomads link up their ships into a moving waterborne city.

A mysterious virus transforms all pre-pubescent children into ravening cannibals but spares adults.

A mysterious virus transforms all adults into ravening cannibals but spares children (who become susceptible at adolescence).

A mysterious virus transforms all animals into ravening man-eaters but spares humans.

A mysterious virus causes incurable, global insomnia.

A mysterious virus causes incurable, global muteness.

A mysterious virus causes incurable, global euphoria.

A panacea is developed that cures all known diseases overnight, with potentially disastrous consequences for the world population.

A city of elementals is organized according to a brutal caste structure.

Lucifer wins the War in Heaven.

Kingdom ruled by sentient carnivorous plants.

Fascist werewolves fight vampire communists (or reverse to taste)

Interplanetary freak-show tours the galactic frontier.

Orc/goblin warlords rule known world; all other races extinct, enslaved, or driven underground.

North America pulls an Atlantis and sinks into the sea, but the rest of the world remains high and dry.

Condemned souls in Hell revolt against demonic overlords, establishing a Republic of the Damned (this one might be my favourite…).

Mummified pharaoh accidentally resurrected by European grave-robbers establishes a new empire at the beginning of the nineteenth-century.

Alternate history in which the Commonwealth of England endured.

Long-extinct human race is revivified in alien theme park/zoo/nature preserve.

Alien amazons kidnap all male humans.

Ever played Machinarium?

Magic, reliant on the mysticism and myth that powers it, is disappearing as scientific knowledge tries to explain it.

Dwarfs, elves, gnomes, orcs, etc., are all the result of a galactic object slamming into the planet and mutating the human gene.

The world lacks a moon; winds rip across the land, tides are infrequent but immense, and only the massive forests of sky-scraper tall trees rooted to the earth shelter civilization from the elements.

Everyone is capable of magic, though magic corrupts both physically and mentally with each use - those few who are born incapable of magic are deemed the most pure and revered beings.

The world is contained within the leviathan proportions of a great, galactic being of immeasurable size.

Fishermen leave the waters to fish for the stars in great, airborne ships.

Magic is given by bargaining with an otherworldly being; many barter in immaterial currencies, such as years, luck, faith, and memory.

The world is a post apocalyptic waste where magicians wield staffs reminiscent of radio antennae to siphon off what little "broadcasted" magic is left from the world.

Sentient foodstuffs long to return to their roots, masquerade as the plants and animals of their origin

Man who believes he is a tree presses discrimination charges against local druid for refusing to protect him from invisible lumberjacks

The bear knows all

The war of the sides plagues the duchy, causing the left side of things to fight with the right side of things.