So You Want To/Build Your Own Fictional World

World Building is the process in which an author adds the finer details to his or her setting. Also check out Be Original and The Trope History of the Universe.

Choices, Choices

 * Will your setting be Science Fiction, Fantasy, both at once or neither?
 * Mythology?
 * What are the inhabitants like?
 * Are everyday people useless, minorly flawed (but that's part of their charm), mean, or awesome and/ or terrifying?
 * What are the politics?
 * What technologies have risen to prominence?
 * Is there magic? If so, what are the rules? How does it interact with technology?
 * Accepted morals?
 * Over-all tone?
 * How "hard" or "soft" is it?

Necessary tropes

 * World Building: Why else would you be here? And if you go through such time and effort to forge out the details, show them off.
 * In a World: Not exactly necessary, but introduce your world somehow.
 * Our Monsters Are Different: A new take on an old beastie is essential. For example, The Thing from Another World combined Doppelganger (by way of You Are Who You Eat) with Bizarre Alien Biology, Our Vampires Are Different and Plant Aliens (it's a species of carrot). The 1982 remake, The Thing, traded plant alien and different vampire for Shapeshifter Mashup, to stomach-turning effect. But don't go so overboard that people shout No True Scotsman.
 * Speculative Fiction: While you could build a fantastic world just for Scenery Porn in a Slice of Life story (like Aria), most likely you'll want to explore the implications of living in your world.

Pitfalls

 * It's Been Done. You need to brainstorm long and hard to make your setting truly unique. Or just cherry-pick bits from other works that you like, toss them in the metaphorical pot, and see what boils up. But be sure to boil long and hard so it doesn't look messily ripped off and stuck on with gore.
 * Recycled in Space. Re-making your favorite science fiction AS A FANTASY EPIC! does not cut bait. it doesn't work other way'round either.

Potential Subversions

 * Standard Fantasy Setting: Everybody who thinks "Hey, I can write a fantasy epic!" usually plagarizes Tolkien or D&D, complete with Medieval Stasis. There are comparitively few Weird West fantasy epics, for example, which can be counted on your fingers (Wild Arms and Deadlands are the only ones that springs to mind); Same with Sci-fi/fantasy crossover settings (Warhammer 40000 and Spelljammer).
 * Our Dwarves Are All the Same: Dwarves are usually all short, fat, mildly agoraphobic drunkards who really like mining, eating, drinking, gold, and fighting. The most different dwarves usually get is that "oh, my particular dwarves have Steampunk tech and /or guns because they stink at magic." Done. To. Death.
 * ISO Standard Human Spaceship: Warhammer 40000's human ships are giant, flying gothic cathedrals with weapons mounts tarted up to look like gargoyles. Johnny's ship from Wonton Soup is spherical. There's no reason to go for the standard flying-box-with-rivited-plates look.
 * Spikes of Villainy: Armor types are rather neutral. It'd be great to find out that, just this once, the Tank in the jet black, spined armor with-skull-mask-helmet is one of the good guys.
 * Five Races/Fantasy Axis of Evil. Orcs (Or Orks, however you want to spell it), Drow/Dark Elves, Goblins, Undead and Dark Dwarves (or necromantic humans, whatever) get a bad rap in most stories, simply because of What Measure Is a Non-Human?.Mix it up a little. Stories like Goblins, Yet Another Fantasy Gamer Comic, and Drowtales explore the "Bad guys" side of it. One could also have the Five races/ Axis of evil completely flipped, with noble orcs and brave goblins vs. bloodthirsty humans and ruthless elves, or peaceful Undead fending off religious nutjobs who consider them "Unholy."

Suggested Themes, Plots, and Aesops
Too many, so I'll just toss you the Genres index.

Costume Department

 * Armor and tunics/leggings /skirts work great for almost any setting. You just need to adapt them to fit the setting. Fantasy gets plate or chainmail armor, Sci-fi gets Powered Armor.
 * Those technologies we mentioned? can any of it be implanted into a character?

Casting Department

 * Whatever classic races you want to include, and any new ones you dream up.

Stunt Department

 * A heroic deed here and there in the history to keep it interesting.
 * And scandals,humorous or serious,to add drama or humor.