Doctor Who can do so much, so it's a good idea to balance light and dark, thrilling adventures, Big Emotional Episodes, horror shows and so on across a series, rather than having a few similar unconnected adventures back to back. Apparently with the mid-series split this year, the episode order has been juggled so we don't have too many funny ones or too many scary ones in a given block.
So take a look at what you've done before, and what you're planning next, and find a place for something a bit different. This could be a format-bender, or simply something in a different theme. Something like a special guest writer might bring this, but it helps to have variety in mind anyway.
10: This, I was not expecting.
Don't work out a strict formula, but look over the series as a whole, the last few adventures and the plans for later on. Where have the characters gone and what have they done? Who hasn't had a spotlight episode, assuming their player wants one? How exciting/scary/funny/weird have the episodes been lately? What kinds of monsters, villains and threats have you featured? What off-the-wall ideas have you come up with lately? Do a bit of free association, talk over possibilities with your players, or grab two random books off your gaming or novel shelf and see what kind of setting would emerge if you slam them into each other...
Example: The Unspeakables
After the gloomy On The Bridge, The War That Never Was and The Shattered World, and before the looming series finale, time for something a bit lighter. Maybe not an out-and-out comedy but certainly more of a laugh.
The Doctor takes Emily and Richard to 1930s Chicago, tracking an alien signal, and they find a gang of alien racketeers muscling in on the bootlegging business. Lovecraft and Untouchables references collide as lizard men in trilbies chase the travellers with Tommy guns.