The empire of glass

Secret agent Will Shakespeare, visiting the locations of his plays while on His Majesty's Secret Service.
Aliens that turned the Roanoke colonists into the pieces of a bomb, to explode if they're ever gathered together in the same location.
The Doctor and his companions, called to 17th century Venice to negotiate an alien disarmament treaty, and meeting Galileo along the way.

Any one of these would have been a good plot for a Who novel. All three of them leaves you with a bit of a mess, plot-wise.