My suggestion is that fuel should be introduced to movecraft as a way of powering the engine (furnace) of all vessels, including Zeppelins, Ships, Planes, Tanks, Trains, Trucks and etc.
Firstly, from a realistic standpoint, fuel is widely used in real life for powering vehicles and it just doesn’t make sense why it couldn’t be used in Nations for the same purpose.
Secondly, the fuel market has been heavily stagnant in a long time and thus, the economy of the server is struggling currently. If fuel becomes more valuable for transportation, it will open many doors for a more engaging economic gameplay and experience, possibly even bringing some much needed calming effect on the current state of constant sieges and wars.
Fuel in my opinion should act as regular coal or charcoal, except it should be giving alot more distance traveled per bucket than regular coal and/or should be making the movecraft move slightly faster (for example by 20 or 30%) than coal. Introducing this mechanic in the game will also slow down the rapid mining of coal and eventual exhaustion of the material, leading to a hyperinflation, as well as it will introduce a more easy, pleasant and realistic travelling for the players