Are you going to include different levels of difficult? Hard, very hard, extreme, impossible?
I’m a bit afraid that without AI it will be easy to lose motivation. I do enjoy just making the network and looking around, but after a while you have a huge network, meaning loads of money, and then its just too easy to get around.
What kept my motivation in RailRoad Tycoon 2 is the fact that you can spend money to purchase the AI and re-make all network, spend money in the stock market and upgrade the entire fleet. Things that are very expensive even when your company have a lot of cash.
Another thing you could include is the possibility to build a headquarters. Each HQ upgrade will unlock new features. That gives a sense of development. Take this as inspiration: http://i48.tinypic.com/2cxyonk.jpg
Just like Traffic Giant, some areas of the map can also be locked, and can only be served after some criteria is reached.
