Before i start this suggestion, i'm gonna be sincere about that i dont have a clear solution to this problem and what i'm going to suggest might be either impossible due to limitation with the plugins involved or that it might be unbalanced.
Nations is a gamemode about war, i know this; but also about economy, and how the economy shifts into a war effort that is exists on weekends and during the time space of regions, due to this most of the time isn't about war, but about working for your town, making and gathering resources, and moving them around the world. This last part is the core of this suggestion, i think that cargo ships and zeppelins are way too vulnerable to being intercepted by faster and more powerful ships, you tried to fix this problem when the armed merchantman was introduced, yet as my own personal opinion and other players around me, it isn't enough for the average CCNet player that dedicates his time on farming, grinding and making all the goods for war to exist. Contracts have lost a lot of relevance because most of the time, the risk in trying to deliver the items is too high, you might end up getting sunk, loosing your cargo and having to pay for the collateral because of not finishing the contract.
Here i start suggesting stuff, add a new status to contracts, once you sign it up, you have time to gather the resources, then you pilot your cargo ship or zeppelin and do (for example) /contracts startdeliver this gives you (for example, maybe the time depends on how far the deliver point is) 2 hours of inmunity in the ship you are piloting, giving you enough time to deliver and comeback to your town, and removing you from /contacts to prevent players wasting their time trying to hunt you down; Will this make the armed merchantman useless? I don't think so, it will still be used to move goods between towns that aren't contracts, like either pots, guns, ammo, sets, etc.
This is one idea i had, i know that Nations is about war, and in war, tracking down your enemies supply lines is a war strategy, but i dont see in real life US Naval ships sunking down Chinese ships for no reason. Thanks for reading.