There is a canal-like feature: A city on a one-tile-wide isthmus, a city and a fort on a two-tile-wide isthmus, or a city and two forts on a three-tile-wide isthmus, although the last isn't ideal because the city won't be coastal. It also works with any combination of lakes and inner seas. You may or may not be able to extend it farther using citadels, but I haven't tried and I wouldn't count on it.
I think most people would agree that this is enough; it's not too restrictive, because you have some options, but not too long to be unrealistic. I don't know whether that map would ever have such a possibility though.