Starting off with the Harry Potter series, the title character of which has inspired more than a few boys and men of all ages to don taped glasses and a lightning-shaped scar, here are some costume ideas which go a little beyond the standard Harry-and-Hermione Party City costumes:
Bellatrix Lestrange (Harry Potter)
Creepy and totally fun to act out, Voldemort's best and last (and only female) lieutenant would make for an awesome and surprisingly easy costume choice. For those of us gifted with wild, unruly hair, the hairstyle requires nothing more than getting out of bed in the morning-- or a particularly zany wig, if your hair tends to lie nicely. A long, tattered dress and a slightly crooked wand would complete the look, though a dagger-shaped necklace wouldn't hurt.
More bonuses: long, tapered fingernails and a good madwoman laugh. Or maybe some ripped Azkaban prison clothes could provide a different look altogether. Also worth considering: a homemark Dark Mark! (Well... if you know somebody who can draw better than the person who drew this one.)
Dobby the House Elf (Harry Potter)
I never expected to have a chance to write his name in creepy Halloween font. That said, I've never personally known anyone to dress up as Dobby. This one might be more of a challenge: you'd need some kind of big, floppy ears and a very large pillowcase to wear, unless you were going for a post-Chamber of Secrets Dobby and wanted to spring for a half-dozen hats and mismatched colorful socks. Much practice on a high-pitched voice would be necessary.
Moving on from HP, what about the more adult fantasy phenomenon, A Game of Thrones?
Arya Stark, Jon Snow, Cersei Lannister, any number of other characters
I could definitely see a pre-teen or teenage girl having fun with the tomboyish and rebellious character of Arya Stark-- maybe two sisters could go as Arya and her more lady-like sister Sansa. I don't know if enough younger people watch the series or read the books, but maybe adult women could pull it off as well. For Arya, I'm thinking scruffy short hair (or long hair, she has long hair at the beginning of the series), a sword called Needle, and a general lack of deference to all authority. 
Alternatively, if your dog is too big or just too furious to be a Toto, it could be Ghost the direwolf to your Jon Snow. For Jon, you'd need some dark Night's Watch-looking armor, tall boots, a sword (of course, with a pointy end), and a curly black wig. Dark stubble is a bonus.
As far as other Game of Thrones characters go, Cersei Lannister would require a fashionable medieval dress, long blond wig, and one basic facial expression. The malevolent and sadistic teenage King Joffrey would also be a great one: it's all in the disgustingly selfish attitude.
From the Percy Jackson and the Olympians series
There are an endless amount of options within this popular YA book series, but there is also the issue that the books aren't popular enough to be recognized by everyone, especially every adult. A few years ago, my sister and some of her friends made orange Camp Half-Blood t-shirts for Halloween and, unfortunately, their costumes led to more puzzled questions than anything. To do Percy Jackson or one of his demi-god friends right, you'd need some god or goddess-like attributes. Think a stuffed owl on your shoulder for a daughter of Athena, or a plastic trident and shield for a son of Poseidon. The huntresses of Artemis would be a good group costume-- a troupe of huntresses--or maybe even hunters-- with bows and arrows slung over their shoulder, wearing blue jeans and street clothes to be accurate to the books.
From Various Other Books
The Twilight-inspired costume I'd most like to see in person isn't another hail-gel-greased Edward Cullen. Little Jane, played so well by Dakota Fanning in her brief appearances in the movie series, would make for a fun costume. Black hood and a creepy pendant necklace (have no idea what the pendant actually is). Then you just need a sinisterly blank expression, some pale makeup, and maybe red contacts (if you were trying for perfection). The worst thing about this literary costume idea: probably few people would guess who you are despite Twilight's renown. 
The Other Mother from Coraline is one of those costumes I'd just love to try. It would take button eyes, of course-- not sure what would be the best way to work that out in a way that you could see, though I'm sure people have done it. And then a long black dress, black hair, and some long black fingernails to complete the creepy look. The button eyes alone would be so eerie! 
Creepy Count Olaf from The Series of Unfortunate Events would be a pretty recognizable costume thanks to the movie adaptation with Jim Carrey portraying him. I would love to see some guy pull off the perfect Count Olaf-- the sinister yet vaguely silly mono-brow and, of course, the eye-shaped tattoo on his ankle. A pinstriped suit and jacket would be the perfect clothes to finish the costume... and bonus points for dressing up like one of Olaf's many bad disguises. Count Olaf and his ever-fashionable, ridiculous girlfriend Esme Squalor might be a good couple costume, too.
