Charlie McGee - For those who don't remember, Firestarter from the Stephen King book/movie/series. Even have her played by the adult Drew Barrymore. Play her as somewhat cynical, if soft hearted and don't show anything special about her until the 2nd act or so when she cuts loose.
Hannibal Lecter - Difficult to keep a leash on, but given the proper McGuffin, he could be coerced into serving the League in exchange for his 'freedom'. Definately a dark character who'd only be genuinely helpful when it suited him.
Chief Deputy Marshal Samuel Gerard - Tommy Lee Jones' character from The Fugitive and US Marshalls. Probably as a bystander and not a League character. Someone who fills them in on what's going on or serves as a Liason for the US Government.
Det. Kathy Mallory - Main character in a series of mystery novels by Carol O'Connell. If you've not read Mallory's Oracle, you owe it to yourself to do so. Kathy is a Detective for the NYPD who's incredibly intelligent, ruthless, amoral, and emotional dysfunctional. As a 10 year old, she was adopted by a NYPD Lt. and raised by him and his wife and she judges morality by what would have made her adopted mother cry. Brilliant character, although one that would be difficult for an actress to pull off.
Sydney Prescott from the Scream series of films might not be a bad choice. She's certainly had her experience in dealing with demented psychos.
Nikita from Le Femme Nikita might be popular enough to work with and the character has been through a number of revisions already.
Dr. Who - The time machine is probably too big a plot device, although one incarnation of the doctor was bound to the earth.
John Smith - The Psychic from the Dead Zone. Another Stephen King character, but he's a popular author and writes in the proper power strata.
Just a few more ideas,
Jesse