To make this more of a MMO, you need something that will get people to work together in order to accomplish something. MMO's are all about making and maintaining social bonds, and providing them with the required social networking tools to do so. The things you do in your city need to have an impact on others, indeed the whole planet, as well as being meaningful. It is not enough to say "I will place this interstate highway here," if it has no real impact on anything. Being able to say "these are our cities, and we worked hard to build them, and now everyone can see what we have achieved" is important to mayors.
I am just speaking on general terms here, but what you want people to say is, "I need to logon today and work with Mr. A, Mr. B, and Miss C, and do X, Y and hopefully Z today so that my city will benefit profitably, and the thing I am trying to accomplish with the 5 other players will get done so that we will increase our influence, wealth, and power and fame on this planet, and other cities will hear about us, and rely on us for our products, services, infrastructure while we kick back, watch the money roll in and use it to buy really cool stuff.
As MMO, I really see no diverse and interesting political landscape at all. There is no group of cities I can join to try and achieve a common goal of interest. There should be some kind of race between cities for companies and jobs. There needs more opportunities and reasons for players to get involved with each other.
Take trains for example; one should not be able to ship freight to a city that does not have a train system. Yet, airports can fly citizens to cities that do not have airports. Planet Geography needs to be factored. How is it that my land locked city can get food from across the ocean without having to pay the city by the ocean for the use of its harbor? And if we all use harbor A, because it is cheap that the mayor charges, but he then experiences gridlock, perhaps it would be better to pay much more for a less used harbor to ship/get the goods I want, need, and desire.
A player run economy, whose market prices are determined by supply and demand, would be nice, not just OC unlimited resource use and payouts at fixed prices would be nice. And since we don't have space travel, territories instead of planets would be great, opening all kinds of possibilities for player interaction.
Anyway, I read some good ideas in this thread so far that would accomplish some of these things.
"Everything that can be invented has been invented."
Commissioner of the U.S. Patent Office Charles H. Duell, 1899