I think a cool feature would be trains, they'd provide a faster way to transport units accross your territory, especially if those troops cannot be airlifted.
Essentially, you'd be given the ability to purchase rail line for any of your provinces. In order for the rail line to be valid, there must be another rail line in an adjacent province. Rail lines purchased in a bunch of adjacent provinces will automatically connect to each other and make up a "rail system".
Similar to boarding an airplane or a ship, the player would need to wait a certain amount of time for the unit(s) would need to "embark" onto the train. Once embarked, the train would then travel at a speed of ~5.0 (less than half the speed of a jet?), and traverse along the rail system toward the location designated by the player. Once the train reaches the province (with a rail system) closest to the designated location, the units would then "disembark" for a certain period of time.
Another layer of complexity to consider, would be that there can only be one train on a particular rail system at a time. This means that in addition to the "time to embark", there may also be a state where the units are "Waiting for train" to arrive.

So in a nutshell, the pros:
-Transport units significantly faster by land
-Provides a faster alternative for non-airliftable units.

Cons:
-Can only utilize this within your own territory.
-Must wait for units to embark/disembark
-Possibly must wait for train to arrive, as it can only be in one place at a time.
-Rail system can be captured/destroyed