We've talked about assassins, and I have an idea for a sorta similar -yet different- "special unit". Wouldn't it be cool, if we could hire some nasty creatures and send them to an enemy city to cause some trouble? Like they should try to burn down some buildings and stuff like that. These units could move undetected on the strategic map because of their stealth ability, just like the assassins. [...Of course there should be global/local for a specific city spells which will "unhide" these stealthed units.]
PS. I couldn't come up with a better name for this special unit, so I named it vandal. If you have a better word for it, feel free to post about it.
*edit* Thanks for Pawels for the "Saboteur" idea with regard to the unit's name.