One of the reasons I was initially excited about TF2 was the inclusion of ships and harbor terminals as a transport mode. Initially I was hoping that this would include the ability to build navigation infrastructures like canals, locks, boats lifts, dams, and aqueducts. Unfortunately, several major updates into TF2 this hasn't come to pass...
Nevertheless, I wonder how much more could could be added from a modding perspective? For example, in the midwest USA and many other commercial riverways around the world, barges are marshalled and hauled by towboats in more or less exactly the same manner as freight trains are organized. On the lower Miss. barge tows can be made up of over 40 individual barges with massive line haul pushboats functioning as a locomotive. Further upriver the tows are limited to 15 barges max due to the lock sizes. It would be cool if we could build barge tows in a similar manner to the freight train consists in TF2. With a number of barge types and towboats/tugs to choose from. Options could start with keelboats or mule-hauled wooden canal barges and move on to steamboats by the 1850s before conversion to diesel and steel construction in the present day.