transport network. Bridge, Tunnel, Highway, roads, railroads, canals.

How great would it be to allow the construction of transport networks?
Some maps require you to drive around a lake, mountian or pit. This is time consuming and costs lots of fuel.

Roads & highways:
Roads should give you an extra challenge and it will add a new feature to the game than gives the player a whole new ambition and experience of the game.
In my vision roads should be buildable on the surface, in tunnels, but also in elevated from so you can cross over bussy intrersections of transport hubs.

Rail & Canals
Trains are know for transfering vast amounts of goods between cargo hubs. Adding ships with the ability to use waterways constructed between retaining wals or actual canals would be a nice addon, also ships could help you at building artificial islands, or dredge soil or resourches from underwater.
Who know in the future.. Underwater constructions.