Salt is one of the oldest exchangeable resources, and retained its value until the end of modern times. In addition, salt reduces the incidence of certain diseases - especially food poisoning, parasites, etc.
But this valuable resource is not represented in the game.
For a very long time in Europe, the main method of obtaining salt was salt production - evaporation of salt from sea or groundwater from salt marshes.
In fact, it is necessary to add a new class of soils - “salt soils” and a class of new deposits - salt - but these deposits should be extremely rare 1-2, depending on the size of the map.
To obtain salt, you need to enter two buildings into the branch - Saltworks and Salt Mines
Saltworks - Building T1, essentially a barn with a boiler in which water is boiled from the salt marsh. (consumes wood or coal)
Salt Mines is a T2 building that opens with other mines.
associated buildings
Drying House - Building T1
Allows you to dry fruits; after access to salt, you can prepare dried meat and fish.
The pickled house is already in the game, just expand the range - after access to salt and after building the “cooper’s workshop” allows you to prepare corned beef and salted fish.
Or introduce a special building T2 for salting meat and fish.
The crossing of resources chains between mining and food production sounds like an interesting addition.
In practice though… the probable game mechanics issue is likely going to be similar to preserves (which do a mechanically similar thing but with sand), in that the benefit of a bit more shelf life is not at all worth the significant expansion of manpower required. With the addition of drought years the value of preservation might go up a bit, but even then so long as you’re not actually losing people to starvation then taking an occasional hit to worker productivity from a bit of hunger is probably still going to be far more efficient than investing extra workers into the resource chains for preservation.
Salt can be made the same essential product as water. - Deficiency leads to disease.
You can do something a little smarter
Introduce a category such as cultural and climatic priority and configure it when creating a map - which will determine priorities in the choice of food production.
Add terrain types: sea coast, island or river valley, dry - with your own diet selection priorities
Moreover, it is not clear why fishing is limited only to the coast.
You can do the construction of boats, piers, sea trade, production of hemp, nets, ropes, canvas.
We are now arguing that it is economically irrational, impractical, but in the Middle Ages a person had geographically limited resources - he ate what he could grow or catch or take away.
More specifically, before railroads and powered vehicles, everybody on the planet pretty much ate what they could obtain locally or have delivered by boat or ship. Traffic over land was limited to either small quantities packed on a man’s or animal’s back, too small to affect food supply, or wheeled vehicles drawn by draft animals. Since the draft animals had to be fed and wheeled vehicles needed some kind of a road, that limited effective land transport to about 100 - 150 kilometers, after which the draft animals had eaten the equivalent of most of your cargo: zero-sum transportation.
Adding food and major resources from far away would, realistically, require ships, docks, and all the infrastructure of water transport, which they’ve more or less said is too much work to get it before Release.
Given the number of times people have asked for more interaction with water features, though, I suspect a Water DLC with bridges, Fishing Boats, a ship/boat-bourne Trader or two, docks and dockside warehouses, and even water-baed industry like a Salt Works is possible.