I'm totally with LichKing on this. For full theme, you only want certain types of units able to be produced in certain areas. And that becomes a feature of a city. A mountainous city may only produce Goblins, but it's ABILITY to pay to produce Dragons is a big draw of the town.
This should be an option of course. But you just need two columns next to each unit in the city's list. PRODUCES this unit, and ABLE TO PRODUCE this unit. Then a universal select/unselect all.
I also like the option of 'no pillage' to be built into the map -- although that's a popular tactic by some players, so it would require so play adjustment.