It seems like the most obvious mash-up of all time in retrospect. But it took the Hillywood Show to finally, finally bring us the Tenth Doctor performing Rocky Horror Picture Show's quintessential song "The Time Warp." Kismet!
I believe Torchwood's Captain Jack Harkness singing "Rose Tint My World" should be next on the agenda, don't you?
Thanks to Dalek_Dave for the tip!