Most cross over events that I've seen are very similar shows, often from the same production studios anyway, like two different versions of CSI coming together in a couple of episodes, what cross over events would you like to see? Would you go for broadly similar, like say Supernatural and Buffy The Vampire Slayer, or would you go for completely different genres, like House MD and Firefly?

Post your cross over event and a sentence as to why it would be so awesome, possibly how it happened.