It would be a complete nightmare to mod, I don' think you could make the cities uniue without basically redesigning in the game and while you could add in special buildings (St. Peters, Dome of the Rock) as one off you would have real problems with footprinting, for example St Peter's (to scale) is far larger than any of the buildings (including the Huge Cathedrals) in the vannilla game, lovely idea but very very difficult, and most people wouldn't notice/wouldn't care, that's probably why there aren't in the vannilla game.