1930/40s Serial Adventure - An Indiana Jones style adventure along the lines of Raiders of the Lost Ark with booby traps, gung-ho grave robbers, desert/jungle chases and evil Nazis explorers.
1700s Piracy - Pirates, privateers, pirate ports, hidden coves, Royal Navy, the French and treasure hordes. Can't go wrong.
The Conquistadors - There has been an popular Aztec story from the Hartnel era, but we can come back to the Aztecs and Incas when the Spanish come round to kicking their doors down (the dramatic story would work without aliens).
Elizabeth I - We can find out how the Doctor got on the wrong side of her in her youth.

Medeval Europe - Something similar to the hinted at Charlemagne story?