The Ski hill would be a great guess...or perhaps even extending the city past the tunnel or expanding the current outer-most boundaries to allow further expansions.
I am wondering if the mayors may be able to "Like" other cities that we visit and the more "Likes" you get perhaps amount to simcash bonuses. I see a lot of cool cities and would like to congratulate the mayors but have no means of communication to them. Just a thought.
