Hmmm... Sherlock Holmes/Torchwood crossover, where Holmes is frankly not impressed with Torchwood's investigative procedures.

(Extra points if you have conversations with Watson and the Torchwood gang, where they just assume he's shagging Holmes because they can't imagine a situation where the office juniors aren't sleeping with the boss. Or Jack talking to Sherlock about the same thing)

or

Doctor Who/Sherlock Holmes, where the Doctor (Ten) and Holmes meet in a bar/pub/somewhere and discuss what it's like falling for your arch-enemy.