Okay, I've been wanting canal improvements for sooo long, and BNW would make the argument even better! Think about it, trade routes over the ocean could really be improved with canals (think Panama in real life). And if you have a canal in your territory, your open borders agreement would be worth more money (in trade) to others who could use it for their own overseas trade routes. If the limit were two connected land tiles maximum length, I don't think it would be overpowered. After all, you have to give up tile yields if it goes nearby a city.