I'm a firm believer in loose interpretations of themes, so anything that messes with the concept of gender is fine here--canonically male characters who were always female and vice versa, characters who have been changed suddenly into the opposite sex by whatever means you choose, transsexual characters, and, of course, characters who play with the lighter side of gender roles by doing drag or just occasionally wearing women or men's underwear.
Please be kind to the code monkeys and remember the posting format: Your Fandom or Crossover, Your Pairing, "Your prompt". One prompt per comment.
Examples:
Leverage, girl!Alec/Parker, "what is it with women and shoes?"
SGA RPS, David/Jason/Joe, "someone's a pretty pretty princess"
Merlin, Merlin/Arthur, "what do you mean, you don't know how to turn me back?!"
Yes, more than one writer can write to a prompt and yes, you may write something for your own prompt, if you like.
And remember, if nothing here grabs your fancy, you can always head over to the Lonely Prompt List and give a lonely prompt a home.