http://kaction.com/badfanfiction/
You should try it! It's amusing.
Here's some of what I received and my extrapolated thoughts on the ideas...
"Your challenge is to write crossover fanfiction combining X-Files and Big O. The story should use an orgy as a plot device!"
I can actually see this one... the android R.Dorothy Wainwright, being "investigated" -in depth- by Moulder and Scully, until Roger breaks in to rescue her... with a french tickler...
Wow, even I'm surprised by how twisted that is...
NEXT!
"Your challenge is to write crossover fanfiction combining the Little Mermaid and 4chan. The story should use bondage as a plot device!"
Oh pah-leeeese, that's been done a thousand times...
NEXT!
Your challenge is to write crossover fanfiction combining Professional Wrestling and Trigun. The story should use tentacle rape as a plot device!
In this corner we have Merle! Weighing in at 145 pounds of petite female fury! And in the other corner, from the deepest pits of hell, weighing in a 874 pounds of phallic force, we have The Shikima monster Lord! I want this to be a good clean fight above the belt! FIGHT!
I seemingly am only getting the dirty ones... Not that I'm complaining...
NEXT!
Your challenge is to write crossover fanfiction combining 24 and Spongebob Squarepants. The story should use creepy internet fetishes as a plot device!
...I'm... what?... I'm sorry, I just can't do this anymore... I... I... just had an awful... AWFUL idea, that requires me now to go hang myself... goodbye cruel world...