if you had full control of one episode that you could include anything you want in there from old characters to old storylines - what would be in there - you dont need to go into full details with it.
Doctor Who - Some big war happens and the prophecy says the war will stop with three time lords. So the eleventh doctor travels back in time to grab the tenth and ninth doctor, along with their companions - Rose, Martha and Donna. The big war consists of Daleks, Cyberman and the other monsters. That would be so cool. This storyline basically has no plot, but I would love to see them all together.
A crossover Sherlock-doctor who episode with either David Tennant/Ten or Eleven/Matt Smith and Amy and Rory, with George RR.Martin as a main writer along with Davies and Moffat. And Tyrion Lannister shows up somehow. Please.
House MD - they have to treat a sarcastic genius with a mystery ailment; lots of clashes between him and House; really cool disease; plenty of witty dialogue; and at some point House and Wilson must make out.