... okay, not a whole episode, but something cool to happen. Maybe a mid-season thing...
Doctor drops off his companion to go on a mission, do something silly or some other reason...
Moments later, the TARDIS rematerializes and out steps someone completely different. The Next Doctor!
Has to say something like "Well, I had to come now." Companion says, "Why?" Doctor says, "You told me that I did - its already happened. But you can't tell me what happens next. Spoilers."
They go on an adventure and he drops the companion back off at the same place, minutes later.
Next episode, the current Doctor comes back and picks up the Companion.
Tough thing to do is make sure the "Next Doctor" plays the Next Doctor.
Also, while I'm on the Doctor... the Doctor's Daughter needs to show up again soon! She went off planet-saving, so I want her to return to the show.