Pick a cast from any fandom and do a gender/sex swap.
I do think about genderbending for some of my fandoms, though I don't always have a definitive idea for who should portray certain characters. The only fandom that I have the majority of the characters swapped would be Supernatural, because I always imagined what a better show and story it would have been had it been mainly a woman ensemble. There's already a popular fancasting, which I have adopted as well, with some tweaks.
** Amber Heard (specifically from Drive Angry, which is the main reference for this pick) as Dean Winchester
As my alternative, I always kind of thought Katee Sackhoff would be perfect as a girl version of Dean, since her role as Kara Thrace from Battlestar Galactica was quite similar to the character.
** Missy Peregrym (Stick It!) as Sam Winchester
** Bridget Regan (Legend of the Seeker) as Castiel
** Jaime Murray (Warehouse 13) as Crowley
** Jessica Lange (American Horror Story) as Bobby Singer
Oh, and I also have a genderbend fancasting for The Hobbit. At least, for one of the characters. For Thorin I'm thinking of either Stephanie Jacobsen (Terminator: The Sarah Connor Chronicles) or Stephanie Beatriz (Brooklyn Nine-Nine). Aside from their ability to be stoically badass, they both have fabulous hair, which, imho, is a huge contributing factor, heh.