A world made of water, water-faring races, and nothing else. (From Here!)
A seafaring fantasy where your protagonist is not a pirate. Bonus points if s/he’s not in the navy, either.
A sky-faring fantasy where there is no such thing as ground.
A magic system where anything written in red ink becomes true. Bonus points if this magic is commonplace and the world isn’t destroyed.
Create a world where grass grows over a hundred feet tall within an hour of being planted. Plan societies and races around it.
A world consisting of islands floating in the sky. Bonus points if the people there don’t have wings, but have other methods with which to travel from island to island.
A fantasy world advanced enough to parallel Earth in 2012.
Go ahead and write 100 of your own ideas/prompts. This will help you get your creative gears going, as you’ll inevitably think in some depth about each item.
Your protagonist has already lost his confrontation with the antagonist.
Your protagonist commits a crime you personally deem unforgivable–in the beginning of the story. Make him/her likable.