Worldbuilding and action fics have some less-obvious labels. The Blanket Scenario, for one. Two characters. A freezing blizzard. One blanket. This is often a setup for cuddling to stay warm and emotional revelations, but it can also be used for straight-up action of the “like heck will I cuddle with you!” variety.
A Blanket Scenario is generally one of many kinds of One-Shot fics. The writer had one particular idea, wrote the fic to explore it, the end. If you write a good one, be warned; some people just will not read the “one-shot” tag, and keep pestering you for “where’s the rest of it?” So it goes. At least that means they liked the idea?
Missing scenes are also often one-shots. They’re meant to fill in the writer’s idea of how a bit canon skipped could have happened. Since canon essentially “continues” this fic, missing scenes tend to get less pestering for more and more “is this what you really thing happened?” comments.
A casefic may or may not be a one-shot, but it tends to be a longer fic. These can be found in any fandom where the main characters get dragged into investigating things, but are especially prevalent in settings where They Fight Crime, or hunt monsters/ghosts/demons of the week. In essence, a casefic tries to tell a story like an episode of the original show; a Leverage heist, a Detective Conan murder mystery, a Monster of the Week on Buffy the Vampire Slayer. Confession: I love casefics. They tend to have a meaty, satisfying story arc finishing with the good guys triumphant and evil vanquished, at least until next week.
For action and worldbuilding fics that stray further from canon, here are a few interesting categories to hit. BAMF character X is what happens when the writer decides to take a particular character and let them be as awesome as they can be. Character’s shtick is analysis and research? Let them have a Plan for every enemy, or at least improvise one. Character’s power nullifies gravity? The writer tried to think of every possible way to use that besides just “make things float”. Character is a scary necromancer? Let’s have some bad guys get absolutely wrecked by angry ghosts and the walking dead.
There’s a fine line between BAMF and powertrip, but done right you have the character still in the limits of canon and yet wreaking havoc on those who deserve it. Note, BAMF does not have to mean action violence. BAMF political maneuvering and plotting your enemies into corners also counts.
AU, alternate universe, can range from the tiny details of Want of a Nail, like a character turning right instead of left at a crossroads (AKA AU – Canon Divergence) all the way to “every character is actually a mermaid/dragon/youkai in this universe.” AUs can include or overlap with crossover and fusion fics. Crossover, in general, is when characters from two different fandom settings meet and interact. Fusion fics, OTOH, take the characters of one fandom and put them into the roles and settings of another fandom. Star Trek and Star Wars fusion AUs are common, and often fun. Who hasn’t wanted to see what their favorite characters would be like as a Klingon, Vulcan, Jedi or Sith?
Then there are more oddball fanfic categories, like YAHF and crack. YAHF (Yet Another Halloween Fic) is based on a Halloween ep of Buffy the Vampire Slayer, where enchanted costumes turned their wearers into those characters for the night. It’s a fun way to do a semi-crossover without worrying about all the picky details involved in making two different settings fit together. And crack….
Crackfics are what happens when writers have a bit too much sugar and stimulants, and not nearly enough sleep. Take an idea, got to an extreme, let the poor characters deal with it. Anything from “the villain banned ice cream!” to “our Hero accidentally started a harem” to “the Ancient Spirit of Evil has taken up macramé, what do we heroes do now?”
(I’d say party time, we can finally hide out in the library and do research! But that’s me.)
Hopefully these are enough tags to get you started. For the daring, TV Tropes pages cover all kinds of fanfic related terms. But be warned. That site can swallow a fan’s attention for hours. Days! It’s worse than wikiwalks!
But boy is it fun….